Isaac Asimov is my favorite science fiction author. His Foundation novels (particularly the original three) are amazing. He was forced into writing sequels like thirty years after making the original, and the quality isn't quite the same. But Foundation and Earth, which was both the last he made, and the second to last in the series, was excellent. Too bad he died before finishing the final book.
I also highly recommend his short book, The End of Eternity. It can be read as a self contained story, but it ties in with with his Spacer, Robot, and Foundation books real heavily. Plus is an all around great time travel story.