Mmm, there are so many of them. Asimov, Clarke and Heinlein the big three of course ;-) But also people like Ian M Banks, Philip Jose Farmer, Stephen Baxter, Harry Harrison, Theodore Sturgeon etc. But my all-time favourite has to be Philip K Dick. His insight into philosophical and metaphysical questions are just mind-boggling. He had a uniquely paranoid view of the world. Although I don't necessarily agree with him, his views on religion were also very interesting, even bizarre. His novel "Valis" is one of my favourite books! (hehe) Also books like "The Transmigration of Timothy Archer", "The Divine Invasion" and especially "The Three Stigmata of Palmer Eldritch" are particulary freaked out. Oh, and "Do Androids Dream of Electric Sheep" also had some interesting takes on religion.
I can't really say who my favorite sci fi author is, but I really enjoyed all of Robert Charles Wilson's books last year. His books are good sci fi, but they are also very good character stories which is why I kept reading them.