Vonnegut is my favorite. His writing style owns, with the vivid imagery and wonderful storylines followed by a liberal and cleansing moral.
I also really liked the Ender's Game series by Orson Scott Card. First book made me cry. :')
Crichton is a great writer as well. His SciFi is really hardcore because he goes in-depth with how all the events in his novels are able to happen. If you read Prey, you'll see what I mean.
The first writer I really got into was Robert A. Heinlein. I liked a lot of his work, Tunnel in the Sky the most. Stranger in a Strange Land was a very satisfying read and made me think about how a Martian (or any foreigner, really) would be impacted by Christianity because of how popular it is.