Some excellent recommendations in this thread already. Special mention to KB's choice which has to be one of the most inspiring books I've ever read, and is guaranteed to move a stone to tears. Can't wait to see that adaptation.
Here are my recommendations:

Quite simply the best book ever written IMO. I loved Joe Heller's response to a journalist who accused him of not having written a book as good as Catch-22 since. "No, but neither has anyone else." Amen to that Mr Heller!

Iain M Banks comes very close, but for my money Dan Simmons is the king of space opera. Hyperion is a metaphysical sci-fi twist on The Pilgrims Progress, as pilgrims to a far distant temple give accounts of their tales in an effort to better understand why they were chosen for the trip. All against a background of interstellar war and malignant AIs. Read the entire Cantos for the ultimate sci-fi tale. His Ilium and Olympos aren't bad either!

Speaking of Mr Banks, it was a toss-up between this or Wasp Factory but, so as not to spoil the surprise for the book cyclists, I'll go with Crow Road. Banks has a great skill wherein he seems to write about nothing in particular, but does it so well it is great reading. You'd be hard-pressed at times to find a compelling plot in his non-sci-fi works, but they're just so damn well written you'll forgive him.

The movie 300 could have been. Apparently it was this book that was going to tell the story of Thermopylae on the big screen until someone at Legendary Pictures optioned the graphic novel. Much better storytelling and much truer to what happened too probably! Not a crab-clawed man in sight!