As with most sci-fi the author gets loopier in the later books. That being said:

• Dune: masterpiece of philosophy, one of the best books ever put to print
• Dune Messiah: a worthy sequel and must read after the first book; completes Paul’s arc
• Children of Dune: more plot driven than the first, but still thematically rich and entertaining.
• God Emperor of Dune: the most divisive of the books: you love it or you hate it. I am in the love it camp, the book is unhinged and the themes are marvelous. This is where I’d stop a read of the series.
• Chapterhouse and the other (Heretics?): forgettable in my opinion, simply because I’ve forgotten them. Later book fan opinions welcome.
• anything Brian Herbert: not terrible but not awfully good either. Makes for decent light reading I guess, and there’s good lore building in some of the books despite some unforgivable retcons (Agemmemnon, sigh)