What I think are unfilmable:
Peter F. Hamilton's - Night's Dawn Trilogy -- Well over the head of most people. There are some things which do not translate well to a visual production.
Zelazny's Amber series -- Just the dimension changing powers alone would make it impossible. So many details just couldn't be shown in any effect manner for the audience to see. It would be neutered most likely to a single world with chaos at one end and the amberites at the other.
Jack Vance - Alistair series & Emphyrio -- No way could a producer do these justice, and it would go over the heads of most people. Emphyrio would just be neutered and maybe reset in modern day, or a future more limited.
Dan Simmons - Illium/Olympos and the Hyperion Cantos -- Same as the Night's Dawn Trilogy. It's too epic, and there are to many things in it that outright could not be shown without serious controversy in the current political/societies we have now.
Heinlein -
Year of the Jackpot(I think society would be bored by it)
Starship Troopers(we already know this flopped, that the movie and cartoons will never do it justice. Maybe a reboot in 20 years will work but probably not.)
The Door into Summer - Time travel stories seem really hard to do. Those that are well done are few and far between. I feel that this alone would keep it from working in a visual media format and stick to the book.
John Steakley - Armor -- Maybe as a miniseries, but I doubt it would be given the right director/producer.
Andre Norton -
Star Rangers/The Last Planet - It would be heavily modified to make sense. Probably end up like the Wing Commander movie where completely uneccessary elements are added in which ruin it in relation to the book format.
Time Traders - Same as The Door into summer. Maybe a tv series, but I feel it would flop after 1 season and be cut. Also key elements are based on cold war ideologies which aren't really understood by millenials and newer generations.
Crosstime - Same as The Door into Summer
Asimov's Foundation/iRobot universe - Yes they are the same universe if you didn't know. It's too epic. I did like the iRobot movie with Will Smith. The movie easily fits into why humans on earth stopped liking robots and using them. I feel that no one could do it justice.
Alan Dean Foster --
Damned Trilogy - No one could do it justice
Parallelities - It's to confusing for most people to see a multiverse hopping character deal with hopping through the multiverse. See Fringe series for some of the complaints people have. Although Fringe did last 4 seasons, so maybe, maybe not.
Jed the dead - Dead alien that's not so dead, presented as a ventriloquist's dummy, just wouldn't be successful. The story is good, but people I feel would be turned off by some of the elements used.
Robert Forward's Cametlot 30k -- It's a good story, but I feel that how good would be lost on everyone. It definitely would need to be a miniseries if it had any chance of success.
What I think would work fairly easily:
Jack Vance -
Showboat World - Miniseries
Ports of Call/Lurulu - miniseries.
Tales of the Dying Earth - animated feature
Moorcock -
Elric series - Miniseries or even movies, animated or live action
Corum series - miniseries or tv series, animated or live action
Dan Simmons - The Hollow Man - Make a great movie or miniseries or Tv series
Zelazny's Eye of Cat - Movie
Andre Norton - Witch World -- Movies, TV series, miniseries, animated or live, it would work. There is nothing complicated that would ruin translation from written to visual media.
Asimov's iRobot(actual novel) - Yes I know there was already a movie done for this, and no it doesn't follow at all with the book/universe it came from. But the stories and others could be easily made into a miniseries or even a documentary about the dangers and quirks of logic based on the 3 laws. This is just the book itself not the whole universe.
Alan Dean Foster -
Journeys of the Catechist - Get the same people who are making the discworld movies on this, and it would be a success.