1- Dune (Book 1) Best plot and action. It also comes across as the most original, since it's the first.
2- Heretics of Dune (Book 5) Best action other than the first.
3- Children of Dune (Book 3) Revitalizes the story but doesn't stand out as much as 1 or 5.
4- Dune Messiah (Book 2) Good suspense and good new characters, but can't quite live up to number 1.
5- Chapterhouse: Dune (Book 6) Not enough happens, but some questions get answered.
6- God Emperor of Dune (Book 4) Too slow to compete with the rest.