+1 for Heinlein, he's my all-time favorite sci-fi writer. I basically grew up reading his novels. You _might_ not like it though if you don't like libertarian values.
Another good name to add to the list is Robert Sheckley. He's definitely worth checking. And re-checking.
Yet another great one is a Polish writer Stanislaw Lem. He wrote a whole lot of very deep and philosophical (but still quite fun to read) sci-fi books. Your experience may vary depending on a translation quality.
Ah, and don't forget about Kurt Vonnegut. Not exactly sci, but worth trying. Who knows, you might love it.
