I read the "original" 6 novels. I put quotes because I think the original was a trilogy then he added on a few before passing, and before the subsequent books from others (his son,etc.). I enjoyed all of them, but it has been a while so I can't really remember which of the 6 I liked most outside of Dune. Here's a picture of the books on my shelf now actually. My final takeaways of the series was that I was glad to read all 6 of these.
(I'm a bit of a completionist though when it comes to this kind of stuff. Read all 5 of the 'Leatherstocking Tales' by Cooper because I wanted to read Last of the Mohicans and saw that it was a part of a larger series. I read those in story order. Interestingly, LotM was not even my favorite one out of them! Pathfinder was my favorite of the 5, but each novel had its moments.)