I've read the series in it's entirety every year for about the last 15 years. When I first read it, I made several friends read the series so I'd have folks to talk about them with. They all loved the series nearly as much as I did and it's still my favorite thing to read. Here's my opinion:
1. Dune - great for originality and hope but those who stop there are cheating themselves. The whole saga is about the pitfalls of wanting and being a messiah and for all the first book's freshness, it's a story we've seen 1000 times...'The Chosen One.' god.
2. God Emperor - the crux of the whole series. More contemplative than the others, more about human nature and history.
3. Heretics of Dune - very mature writing and great action. Delves into sex and romantic relationships way more than the other books (it was the 70s) but great ass-kicking battles between the Bene Gesserit and their warped enemies the Honored Matres. Introduces fan-favorite, Miles Teg, 'The Bashar'.
4. Chapterhouse: Dune - a counter-piece to Heretics but good in its own right. Best twist of the series and ends on a genuine cliffhanger.
5. Dune Messiah - a decent sequel to Dune that casts Paul in the worst light. Almost the 'Empire Strikes Back' of the series as it ends on a series of downers but it's just that good. Still, very memorable scenes.
6. Children of Dune - not great. Few real twists and Leto is boring. Ghanima totally unexplored. Still, light-years ahead of Brian Herbert's horrific Dune abortions.