HERE! RIGHT HERE! Huge fantasy and sci-fi fan, my whole life.
How has Orson Scott Card not been mentioned yet? For shame~
Pretty much anything he has written is a blast to read. Ender's Game was good, but there's so much more he's written that's excellent. The Lost Gate was good as well, but that's more fantasy.
Anne McCaffery is a top sci-fi writer. Her Dragonriders of Pern is an excellent read as well, but it gets a bit slow and political as the series carries on. The City Who Fought and The Ship That Searched are also excellent books. They both take place in her 'shellperson' universe, where people who would normally die in infancy due to genetic defects are instead put into a "shell" that supports all their bodily needs while their mind is connected to thousands or even millions of electrical systems, such as a space ship, space station, even a capital city on a planet! The 'shellpeople' as they're called, are totally dependent upon the mechanical shell to support them, but they are like technological gods -- better, smarter, and faster than any AI. I love this series. I wish she'd write more of them.