Tolkien
Ursula le Guin
Pratchett
Neil Gaiman...
...almost anything by the above.
I also like:
Storm Constantine - especially her Wraeththu books, in which a freak mutation in a future world causes a new magically gifted hermaphrodite race to start taking over from humankind. (Her books generally have recurring themes of gender/sexual exploration, the occult, fallen angels and vampirism. Very cool.)
C. S. Lewis' Cosmic Trilogy - Out of the Silent Planet, Perelandra and That Hideous Strength. They can get a bit annoying in places if (like me) you're not a Christian, but they're amazing feats of imagination.
Olaf Stapledon - Last and First Men is not so much a novel as a vast future history of the human race. Star Maker is a history of the universe and an exploration of whether there is such a thing as a 'cosmic mind'. He's often overlooked, but an amazing writer.
Also, Michael Moorcock's Dancers at the End of Time trilogy. Hubby's the real Moorcock fan (I think outsider teen boys all want to be Elric!) - I never really got into the other Eternal Champion stuff, but I like this one.
Oh yes, and a wild card but a fave of mine, John Varley's Millennium. Should you happen to have seen the really crap movie of the same name, the book is way better - a weird time-travel story which should not, on any account, be read on plane journeys...
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"Grunge? Isn't that some gross shade of greenish orange?"