I haven’t read the entire Dune series — in fact, I haven’t read past the first volume — but I have read about its decline, which made this question elicit a chuckle:
JMS: You said earlier that your original vision of the Dune saga encompassed only the material in three volumes culminating with Children of Dune. Why did you write more?
It does decline after the first book. That said, I have a soft spot for God-Emperor. Something about a millennium long absolute monarchy seems simultaneously awesome and terrifying.
I liked 5 and 6, but I think I’m more or less alone in that.
2-3 are OK, but 4 is just painful.