I read mostly SF (my motto is read SF and play Fantasy ...), so my list includes more SF than Fantasy, but anyway:
SF
Iain M. Banks: He´s my favourite author. His books are Space Opera at it´s best, very imaginative and with a good portion of humour in it.
Alastair Reynolds: also Space Opera. Very dark and gritty.
Neal Stephenson: Snow Crash is cool, Cryptonomicon is really interesting (and teaches you a lot about encryption methods ...). Didn´t read the baroque cycle, though.
I also like "old school SF": Larry Niven, Ray Bradbury, Philip K. Dick
Some authors are one-hit-wonders in my mind: e.g. William Gibson´s Neuromancer trilogy is great, everything else is merely ok IMHO
Fantasy
Robert E. Howard; The original Conan stories are a fine piece of literature. Howard´s really good technically. Didn´t read anything else of his work yet, but it´s only a matter of time until Solomon Kane or King Kull land on my bookshelf.
Also fine Sword and Sorcery: Karl Edward Wagner´s Kane stories and Moorcocks Elric books
and of course: Tolkien
There are some game-relatéd books, which I like: The Dragonlance Chronicles by Weis/Hickman, most books by Elaine Cunningham, the Azure Bonds trilogy by Jeff Grubb.
Other literature:
I like Franz Kafka. Don´t know if you´ve ever heard of him (he´s a german writer from the early 1900s) and I don´t know if his books are translated into english. Very frightening (not in a horror sense) books. You should check out "Der Prozess" ("The trial"?).