Again, here's someone who finds this question very difficult to answer, I've read so many! Where to begin?
I was thinking about which books I had read again and again, and the first that popped into my head was Hitchhikers, but I honestly think I enjoyed Dirk Gentlys' Holistic Detective Agency more. It's a tough call, it really is.
As the last books were humour, my Stick of Fantasy Gold has to go The Saga of The Exiles by Julian May, although it could be argued that it is both sci-fi and fantasy. Still a great set of books, and then Intervention and The Galactic Milieu Trilogy top it off nicely.
As for pure/hard sci-fi I am leaning towards Consider Phlebus (sp?) by Iain M. Banks. I really like all his Culture novels though, and it was really hard to choose between my first choice and Player of Games which was a truly stunning novel. They kinda remind me of Hamiltons universe with the Edenists and Voidhawks being somewhat similar to the Culture.
Then you get to books that I enjoyed as a kid, and to be brutally honest, and this often baffles people, I think my favourite was All Quiet On The Western Front by... *Googling* ...Erich Maria Remarque. I think it is still being used today in schools the world over, but it needs to be, it is a great book.