CAVEATS, COMMENTS, AND reCOMMENDATIONS
I. CAVEAT (“It Wasn’t Me!”)
Humor should not be dissected,
because nothing lives through dissection.
– Blood's a Rover, Harlan Ellison, Subterranean Press 
We can all agree that we often disagree on what is funny. Humor takes many forms: slapstick, satire, juvenile, sophisticated, droll, ironic, morbid, self-deprecating, caustic, farcical, witty, parodic. [See: 20 Types and Forms of Humor by Marc Nichol]. What one finds hilariously entertaining may cause another to yawn or furrow their brow and say, “I don’t get it,” or worse, become incensed: “That’s not funny!”
What makes you laugh? The Three Stooges slapping one another or Steven Wright’s dry deliver and play-on-words? Or both. A person’s sense of humor, I’ll propose, can be graphed as a parabolic curve. Some are narrow; some broad.
To make it more complex, this curve fluctuates with mood, age/experience, and even the company we are with.
The preceding is all a cautionary note, that our suggestions to you of “underappreciated genre” works of humor “beyond Adams and Pratchett” – and, I hope, yours to us during the Q&A that will follow – may not necessarily make you chuckle or even grin. But if it does… there’s nothing finer than sharing a good laugh.
And which of us on the panel have the best suggestions doesn’t matter. It is not a competition. We laugh at the suggestion (but I laugh more).
II. HUMOROUS SFF: ADAMS AND PRATCHETT
God's Final Message to His Creation: “We apologize for the inconvenience.”
– So Long, and Thanks for All the Fish, Douglas Adams, Harmony Books, 
Susan hated Literature. She'd much prefer to read a good book.
– Soul Music, Terry Pratchett, Victor Gollancz, Ltd., 
What makes the best humorous SFF? They are stories with relatable characters in absurd situations; relevant stories with humorous elements. A good joke may make you laugh, but a good SFF story will make you think, make you see the world, and perhaps yourself, a little differently.
In addition, the great humorists are masters of delivery. What is said is not as important as how one says it. Douglas Adams and Terry Pratchett were masters of this. It is the style of their writing, their wordcraft, that distinguishes them. Both were British farcists (I can imagine both writing a Monty Python-like Hitlerian parody of the term), writing witty moral comedies involving extravagant improbable situations and characters. Of the two, I’d personally give the victor's wreath to Terry Pratchett, perhaps unfairly, because Douglas Adams tragically died so young. In addition to mastering humor through style, Terry I believe gained a greater skill in plotting. After the fourth Discworld novel, each subsequent book can stand alone despite shared characters and setting, but they are also richer when considered as part of the same greater Discworld story.
Nu? Therefore, if you like Adams and Pratchett, the following book series may interest you:
Robert Asprin (& Jody Lynn Nye): MythAdventures – Another Fine Myth  (Book 1 of 21)
“An outrageously tongue-in-cheek series tracing the haps and mis-haps of a young sorcerer named Skeeve and his brusque demon-mentor, Aahz (“Oz?” “No. No relation”). In the course of their travels, they lampoon every done-to-death plot of action-adventure-fantasy literature and cinema.”
Piers Anthony: Xanth – A Spell For Chameleon*(M)  (Book 1 of 41)
Also a "reworked edition" .
Winner of the British Fantasy Society Award 1977. A lively and whimsical interpretation of a genre often criticized for taking itself too seriously. “Piers Anthony's Xanth series is a tongue-firmly-in-cheek affair, filled with awful puns about bad dreams delivered by horses — literal "night mares" — and corny jokes… A Spell for Chameleon is no fairytale, the princess is a crone and the prince's name is Bink. Seriously. Bink. But both Bink and Chameleon found the person who accepted them as they were — the very essence of magic.” [NPR.org]
*(M) misogynism concerns raised – Note: not an uncommon complaint for many a twentieth century work retrospectively assessed .
Christopher Stasheff – Warlock of Gramarye – The Warlock In Spite of Himself  (Book 1 of 13)
A cross-genre series that blends, science fiction, fantasy, romance, mystery, and a little off-beat western at times. Not intentionally written as humor, for the most part, but funny naturally; situational humor, puns, etc.
"Rod Gallowglass is a man of science who does not believe in magic. Gramarye is a world of witches and warlocks. Of strange abilities and phenomena. A world where society mirrors Earth's own Middle Ages, and a world headed for doom. Rod Gallowglass must become a part of the local fabric to save the world from both itself and external forces that threaten its existence. But to do so, he must put aside his own convictions and beliefs, and become a warlock, in spite of himself."
Roger Zelazny and Robert Sheckley: The Millennial Contest – Bring Me The Head of Prince Charming  (Book 1 of 3)
"People feared, back in the Middle Ages, that the world would end with the millennium. They weren’t wrong. It does this every millennium, only nobody notices—except for the Forces of Good and Evil who vie for control of the universe every thousand years. Azzie’s proposal to the Powers of Dark is simple: He will create a Prince Charming and a Sleeping Beauty. In time-honored fairy-tale fashion, the prince will fight his way through numerous perils to reach the side of the spellbound princess—at which point Azzie’s evil twist will ensure that the Powers of Dark will win the grand prize. But even with an unlimited satanic credit card to order up any evil he needs, Azzie’s plan is in trouble from the beginning."
L. Sprague DeCamp – Novaria – The Fallible Fiend  (Book 1 of 6)
"The demon Zdim was happy with his philosophy on the Twelfth Plane, until he was conjured to Prime Plane to serve the human wizard Maldivius. There, to a logical fiend, men seemed wholly irrational. He was ordered to eat the first being to enter a sanctum, then beaten for devouring the wizard's apprentice. But war came to the city of Ir. The role of savior should have gone to a hero out of the legends. But there was only Zdim — and he was fallible."
Ira Neyman – Transdimensional Authority – Welcome to the Multiverse  (Book 1 of 3)
When a dead body is found slumped over a modified transdimensional machine, rookie Noomi Rapier of the Transfdimensional Authority investigates. Her investigation leads to a variety of realities where Noomi comes face-to-face with four very different incarnations of herself, forcing her to consider how the choices she makes and the circumstances into which she is born determine who she is. Ira’s style is at times surreal, even off-the-wall, with the humor flying at you from unexpected angles; he describes it as fractal humor.
For stand-alone novels in the Adams and Pratchett vein:
Philip Jose Farmer (as Kurt Vonnegut's Kilgore Trout) – Venus on the Half Shell*(NP) 
Simon Wagstaff, the last surviving human being after the Earth gets destroyed in a second Great Flood caused by an alien race that goes around the universe cleaning planets. He escapes in an abandoned space craft that just happens to pass by, encounters many strange aliens with strange customs and has strange adventures, while he travels to the most distant corners of the multiverse, to seek out the answers to the questions no one can seem to answer. Sound familiar? All he’s missing is a towel which, after the Great Flood, he is in greater need of one than Arthur Dent; but Venus On the Half Shell was published in 1974, four years before The Hitchhiker’s Guide to the Galaxy first aired as an audio drama on BBC radio. 
*(NP) Not for the prudish. Sexual content as per Kurt Vonnegut's depiction of Kilgore Trout in Vonnegut's own work [e.g. God Bless You, Mr. Rosewater].
Catherynne M. Valente – Space Opera 
“The Hitchhiker's Guide to the Galaxy meets the joy and glamour of Eurovision.”
Isidore Haiblum – The Tsaddik of the Seven Wonders 
“The tsaddik wanders around through time & space, while a wisecracking Retief/James Bond sort of figure from a galactic bureaucracy accidentally rescues a Polish princess. Eventually they all meet to fight an intergalactic real estate conspiracy, culminating in a climactic battle between hordes of demons & time-hopping Hasidim in a Polish castle. 60s psychedelia meets Yiddish humor." - iO9
III. CLASSICS FROM THE GOOD OLD DAYS
The best thing about “the good old days”
Is that we were neither good nor old.
Literary humorous SFF: This is not an oxymoron [a term, I imagine, Pratchett or Anthony would animate as an unintelligent bovine]. I propose two types:
1. Humorous SFF set in the literary worlds of other authors:
L. Sprague DeCamp and Fletcher Pratt (& others) –The Incomplete Enchanter; - Collection – The Mathematics of Magic
Psychologist Harold Shea and his colleagues are propelled back into versions/worlds of a mythic or literary past: Norse, Irish, and Persian mythologies; the Finnish epic Kalevala, the literary worlds of Edmund Spenser’s The Fairie Queene, Ariosto’s Orlando Furioso, Cervantes’ Don Quixote, Shakespeare’s The Tempest, Samuel Coleridge’s Xanadu in his poem Kubla Khan, as well as Frank L. Baum’s Oz and even Edgar Rice Burroughs’ Barsoom, and more. Shea has a terrible time coming to terms with the local customs in worlds where magic and fictional science works.
Link to more about the tales of Harold Shea.
John Myers Myers – Silverlock
From John Adcox’s 2009 Review: "Silverlock is a booklover’s book, sure. But more importantly, it’s fun. There are battles, quests, love lost and won, drinking bouts, and enough adventure to fill a library. Which is, of course, fitting. There are belly laughs a plenty, and songs you’ll ache to sing. (Accompanied by) the bard Golias (a.k.a. Orpheus, Widsith, Amergin, Taliesin, and pretty much every other bard name you can think of from myth and legend), he encounters the witch Circe from Greek myth, Beowulf, Robin Hood, Puck, the Mad Hatter, Oedipus, Hamlet, Pangloss, Don Quixote, Faustopheles, and…well, dozens of other characters from myth, lore, legend, and literature…This isn’t just any island. It’s an allegorical place in the most mythic sense. It’s the Commonwealth of Letters, and it changes you. Chapter by chapter, we see Shandon… tempered and reshaped until, at last… he (transforms from) someone that’s easy to loath… into someone we can admire (and relate to)."
2. Humorous SFF written by literary authors
Mark Twain – A Connecticut Yankee in King Arthur’s Court (public domain)
James Branch Cabell – Jurgen: A Comedy of Justice (public domain) (free audio)
Jurgen is a rogue, and yet a pitiable schlemiel; an aging pawnbroker and his quest for lost love and coming to terms with his mortality. He is also a lascivious rogue who while devil-relieved of his nagging wife, sets off to rescue her. The tale is replete with sexual innuendos, but they are a mere sidebar.
“Cabell’s approach is a unique mixture of extreme romanticism and extreme cynicism, by turns achingly poetic and wildly funny, but ultimately the cynicism tends to win out. Successive chapters in Jurgen are titled “Of Compromises in…” various places, and compromises form the heart of the novel. Despite having the love of stunning beauties – Dorothy la Désirée, Guenevere, Anaïtis the Lady of the Lake and Chloris the hamadryad, and even having the opportunity to love Helen of Troy – it’s his comfortable home and his nagging wife that he wants back. Just not too quickly.” – Nyki Blatchely, Fantasy Faction, 2011.
Kurt Vonnegut Jr. – Slaughterhouse Five and The Sirens of Titan
“Everything was beautiful and nothing hurt.” - Slaughterhouse Five, Delacorte, 
“When a Tralfamadorian sees a corpse, all he thinks is that the dead person is in a bad condition in that particular moment, but that the same person is just fine in plenty of other moments. Now, when I myself hear that somebody is dead, I simply shrug and say what the Tralfamadorians say about dead people, which is: ‘So it goes.’” -Slaughterhouse Five, Delacorte, .
IV. HUMOROUS PLAYFUL PROTAGONISTS WITH STRONG VOICE; WIT AND ACERBIC WISDOM – (ROGUES):
There are tales that promote themselves as humor, and then there are others with moments of humor in standard SFF tales that make us smile and even laugh aloud because we love (sometimes love-hate) the characters.
Jim Butcher – Harry Dresden of The Dresden Files
More a smartass caring wizard than rogue, in my humble opinion.
“The building was on fire, and it wasn’t my fault.” – Blood Rites, Roc, .
“That’s the problem with you nearly immortal types,” I said. “You couldn’t spot a pop culture reference if it skittered up and implanted an embryo down your esophagus.” – Small Favor, Roc, .
Harry Harrison – “Slippery” James Bolivar diGriz of The Stainless Steel Rat
Likable rogue; a thief turn law man.
“A hero who dares to do what we only daydream about. He's a rebel, an outsider. He is intelligent, quick-witted, with a sense of humor, and a seemingly endless supply of great high-tech gadgets. He values his own freedom and individuality, but at the same time he is a person who values life, all life: in ten novels he has only ever killed once, and then only in defense and with great reluctance and regret.” – Paul Tomlinson, 1999.
Keith Laumer – Jame Retief of The Corps Diplomatique
The sane man among rogues and fools. "Retief is a James Bond among timid, ass-covering diplomats; a hilarious satire where the diplomatic team proves its utter inability to negotiate its way out of a paper bag, followed by a James Bond story where Retief smoothly and debonairly accomplishes what entire government task forces could not, followed by more hilarious satire as the team of idiots attempts to claim credit for it."
The Retief tales were inspired by Laumer’s experience working with diplomats while in the United States Foreign Service.
Jack Vance – Cugel the Clever of The Dying Earth
Cugel is a rogue, an antihero. His self-appointed cognomen “the Clever” is akin to President Trump calling himself “the greatest President.” Yet for what befalls Cugel while he plans and schemes, he is both a schlimazel who evokes our sympathy and laughter.
Stephen Brust – Vlad Taltos of Jhereg
“Jhereg is sort of a crime/mystery story. But rather than solve a crime, Vlad must figure out how to perpetrate one... Jhereg reads like a fantastic and slightly off-kilter version of a Golden Age crime story (with the) focus is on Vlad’s ingenuity (with) witty banter, snarky sidekicks, and action.” - Fletcher Vredenburgh, Black Gate.
Link: About the Vlad Taltos series.
2. Other Classic Tales Worth Mentioning:
Roger Zelazny – Trumps of Doom
Among other things, Merlin/Merle must deal with the mess left by his father in the first Chronicles of Amber series.
“I heard a crashing noise. A horned and tusked purple thing went racing along the ridge to my right pursued by a hairless orange-skinned creature with long claws and a forked tail. Both were wailing in different keys. I nodded. It was just one damned thing after another.” Trumps of Doom, , Arbor House, .
Roger Zelazny (again) – A Night in the Lonesome October
Carol Carr – Look, You Think You Got Troubles? and Harlan Ellison – Searching For Kadak
These Carr and Ellison short stories are found in the anthology Wandering Stars by Jack Dann, a collection of Jewish SFF. The style and voice used in telling the tales and, admittedly, the type of self-deprecating yet pointed humor that for me is nostalgic, recalling my relatives when I was a child, made these stories particularly delightful for me.
Frederic Brown – From These Ashes
The original SF master of the short-short story.
Opening hook for the classic Brown short story Knock: “The last man on Earth sat alone in a room. There was a knock on the door …”
Robert Sheckley – Store of the Worlds
“Sheckley’s brand of humor is based on parody, allusion, satire, and word-play with ideas from all walks of popular culture and history, but especially from the conventions and traditions of science fiction and related genres.” – Greg Johnson, SFSite.
“A number of his stories could be called "farces of misunderstanding" -- situations in which two different cultures, or races, or species, are unable to understand each other, usually because of radically different world-views and assumptions.” – David Horwich, Strange Horizons, 2000.
E.g. The Monsters, a biting, very disturbing, social parody on perspectives. Who decides what is moral; and what is "human?"
Harvard Lampoon – Bored of the Rings
The quintessential, and the first and best, Lord of the Rings parody.
V. SAMPLES OF THE NEWEST WAVE:
Peter David – Sir Apropos of Nothing
Catherynne M. Valente – Space Opera
The Unidentified Funny Objects series delivers an annual dose of funny, zany, and unusual science fiction and fantasy stories.
“From satire to situations of pure irony, from fiendishly dark humor to quirky little scenarios, almost every style of humor is covered, which is a feat in of itself.” – Jeremy Szal, Strange Horizons, 2015.
And a shameless promotion (i.e. demonstration of chutzpah) for:
Two authors whose short stories I found delicious in their use of humor. So I selected them to appear in the 3rd and Starlight  anthology (edited by yours truly).
And, since you are so nice, I’ve made the Kindle ebook ***FREE TO DOWNLOAD*** on Amazon during my Arisia visit January 18th and 19th, 2019.