I'll second the vote for Ender's Game (a science fiction classic). I'm rereading the Otherland series that Elvi also mentioned, and it's been good again (though I think it could have fit better in 2-3 books than 4).
One science fiction author I always love to recommend is Vernor Vinge. His last 3 novels have each won top awards (either Hugo or Nebula), and while I'd recommend them for different audiences, they're all really good. A Fire Upon the Deep is a galaxy-spanning adventure with good ideas and interesting aliens. A Deepness in the Sky is in the same "universe," but it's much more character centered and a bit scary in some of its implications. I rate that as his best; it's just really interesting and so well thought out. His most recent Rainbows End is more light-hearted and much more contemporary, but it has a very practical view of where technology is headed near-term.
If you're interested in far-flung speculation about insane tech, I recently ran into Charles Stross. Most of his stuff has been really good, but Accelerando is mind-blowing. It's fairly technical and way out there, but it's sooo full of interesting ideas.
I have lots of others I could recommend if people are interested. ^_^;;
For fantasy, I haven't read much aside from Tolkien and Pratchett (who's great for comedy fantasy), but I'd recommend The Chronicles of Amber by Roger Zelazny. I think that comes Elvi-recommended too. ^_^v