I'm a huge fan of the original Foundation trilogy by Asimov (Foundation, Foundation & Empire, Second Foundation).
For fantasy, I loved the Hobb Farseer trilogy (Assassin's Apprentice, Royal Assassin, Assassin's Quest). However, people seem to have strong reactions to this series, either love or hate.
And then there's the ever-popular Ender's Game, which I read for the first time when I was about 20.
I've heard great things about Joe Abercrombie's books, but I haven't really gotten into them.