Fantasy - wise, I'd recommend Raymond Feist's Riftwar series. Start with Magician and go from there. Joel Rosenberg's Guardians of the Flame series is good too, and if you've not read Tolkien yet, it's time.
For sci-fi, I'd recommend Niven & Pournelle's Mote in God's Eye and Peter Hamiltons Night's Dawn trilogy as starters. I could rattle off dozens but I'm on my phone and those are some of my all time favourites.
