Pretty much anything I've read by Card has been really good. The entire Ender series and the related stories. Homecoming was a bit bizarre, but good. Probably my favorite ever by anybody would be Pastwatch. Brilliant book.
The Dune series, of course.
When I was in college, I really liked Niven's Ringworld series and the Integral Trees a lot. That was so long ago, I have no idea whether they are really as good as I remember, but I liked them back then.
Piers Anthony's Adept series was also an enjoyable scifi/fantasy crossover that I liked a lot.