I just read some recent posts on another site, listing the 50 greatest sci-fi books of all time. So here's a (shorter) list:
1. JRR Tolkien: Lord of the Rings.
Amazing books. Yes, they can be a little slow. But really, I don't think anything from the 20th century can beat it. Start out with The Hobbit...not as well written, but a bit easier to read, and a good introduction to the series.
2. George RR Martin: A Song of Ice and Fire
A lot like Tolkien in that it's high fantasy, very well written, great characters, great plot. Different than Tolkien though--more graphic (so readers beware), more gritty, more realistic. This series will also become a classic...eventually. He needs to finish the series first though...
3. Robin Hobb: Assassin series
Good fantasy. None of the dorkiness of pop fantasy, and well-written and interesting. The Mad Ship series I didn't really care for, but the follow-up to the Assassin series was good. She's currently working on a fourth series...I'll probably wait until it's done to read it, but it's definitely on my list.
4. Orson Scott Card: Ender's Game series
Ender's Game is a great book. The rest of the series is more science fiction and more political games, but they're still good. Surprisingly, the follow-up series, Ender's Shadow, is also excellent. Fun to read, and the situations and characters provide a lot to think about.
5. Frank Herbert: Dune
The sequels just get stranger and stranger, but Dune is a great novel about life on a desert planet. Classic science fiction.
I'll update this list in the future. If you have any recommendations, let me know.