I read Dune only once, but I absolutely loved it. It had a tremendous impact on my taste in sci-fi, and has become the gold standard by which I evaluate every other sci-fi story.
The sequels, meanwhile, didn’t impress me so much. Dune Messiah was good, but it didn’t floor me like it’s predecessor, and Children of Dune was only fair. God Emperor wasn’t bad — it was better than Children — but the cynicism was so ripe, it turned me off from finishing it.