I rarely run into anybody who read all of the original Dune series and liked it. It's has been a long time for me, but I remember being really into a lot of the later story elements with Duncan Idaho clones and the Honored Matres. Hmmm, kinda want to read it again. Maybe in the future.So many.
If I had to pick a single series though it would be Dune by Frank Herbert.
Especially the third and last book of the series.