These would be impossible, or at least really hard, to adapt to screen.
My picks:
Finnegan's Wake
The Mote In God's Eye
Through A Scanner Darkly
Gravity's Rainbow
Ulysses
Invisible Monsters
The Crying Of Lot 49
Um, they're currently working on "Through a scanner darkly". And I know the rights to "Invisible Monsters" have been optioned by some studio, but they're working on Palahniuk's "Choke" first.
GERALD'S GAME
The Mote in God's Eye would amke for an amazing film, but it's not a case of special effects, it's a case of finding a director and a movie company willing to pull out all the stops and be willing not to moan about budget cost...so in terms if decent companies willing do so, the "Moties" would be unfilmable.
Two sets of books that would be unfilmable would be The Chronicles of Thoams Covenant by Stephen Donaldson....and Necroscope by Brian Lumley.
Necroscope would be amazing, but no doubt if it were made into a film, it be changed so it would be set in America (like James Herberts' The Rats...and yes there are two versions of this film and both are appalling!) and too many compromises in plot due to the simple fact that many people "May not get it"
Although RAINBOW SIX translates very well to film, it's too long to be made into a not-five-hour movie. Also, everything in it is important to the plot, so no removing incidents.
"BATEMAN!!!!!"
Dune - It's been tried more than once, but so much of the book is internalized to the characters it nigh impossible to get it right.
The ScFi mini-series was much closer than the David Lynch movie.
They Thirst by Robert McCammon. I believe it's set in Los Angeles, and by the end of the book, almost everyone has been turned into vampires, then the entire city is destroyed, leaving a giant crater. I suppose there's nothing in the book that isn't technically do-able, but I doubt that any movie could successfully capture the feeling of the entire city being full of vampires.
I second the Chronicles of Thomas Covenant. Revelstone under seige would be nothing but CGI, although I would be curious to see someone's concept of an ur-vile. Unfortunately I foresee some director lumping the whole saga into a two-to-three hour MTV blip and missing the whole point of Donaldson's tale.
I agree with JohnL that nothing is technically unfilmable--even War and Peace has been done to a technical tee (1973 BBC miniseries at 13 hours running time)--but what transcribes to the screen will always be vastly inferior to what the reader's mind can conjure.
"It" by Stephen King. The movie that came out was a total "bore".
Gotta agree with you, Ellie. "It"...Tim Curry was great but the way the character came off, when it said the line about being the eater of children and how powerful it thought it was for doing so, you just with Bruce Campbell would turn up to shut it face!
The Stand...the ending in the televised version built up to the great fight between good and evil...building up, building up, BUILDING UP AND...scene changes from the good guys on their way to battle to Gary Sinese coming back aying that good won. What a jip!
Onionhead raised some of the points I didn't have time to mention. Another point would be the giants...CGI. ur-Viles and Waynhim...CGI. The Ravers...CGI. And you can guarantee Thomas Covenant will be played by some gimp. Shame about his age, as clint Eastwood would have been excellent as the grumpy anti-hero!