Dune seconded with true fanboy gusto.
I remember really enjoying
2001/2010 as well - classics of the genre, but deservedly so. Someone mentioned
Snow Crash I think, which I mean to read at some point - Stephenson is great, although I've not read any of his novels that are actually futuristic but even his 17th/18th century novels are so packed full of science and mathematics (in the best kind of way) that they are in a sense 'sci-fi'.
Cryptonomicon (set in WW2/present day) is probably better, though.
And don't let the idea that Philip Pullman's 'His Dark Materials' trilogy are "kids' books" put you off, they're really not and they're fantastic. Includes the best Catholicism-bashing this side of (ahem) Dan Brown.
I quite enjoyed Will Self's foray into the genre,
The Book Of Dave, though it's arguable whether the book counts as sci-fi just because some portions of it are set in the future (England as a post-eco-pocalypse mediaeval theocracy).
Also, this thread reminds me of something a mate of mine asked me the other day, which was "Do you read any non-sci-fi?" Yeah, because why would a scientist be interested in that obscure niche genre, non-sci-fi?