I've read his first novel of the Foundation Trilogy (or is it a quadrilogy now?) which I thought was pretty cool, and I, Robot, which I thought was fantastic. Too bad they butchered the story in the Will Smith film -- the novel is really excellent science fiction.
To anyone wanting to get into sci-fi, I always tell them to start with the ABCs of the genre: Asimov, Bradbury, and Clarke.
... I should probably add Herbert and Dick in there, but it would ruin my nice little acronym.