I like the first 3 books about equally. God emperor is probably my least favorite of the main series. I still think its a good book, but it was a bit of an awkward transition in timeframe to me. The first 3 books took place in a few decades, then suddenly its 1000 years later and Leto has become a giant worm dictator. It just felt weird to me.
A novel primarily featuring a giant sandworm dictator waxing philosophical is as beautiful an idea for a book as it gets. The rest of the sequels aren't as creative, though all of Frank's works are very good.