Book 1-3 of Dune are masterpieces IMO. Book 4 was still good although I didn’t enjoy it as much as the trilogy. But I still consider it part of the same overall “Leto/Paul arc”.

Book 5-6 were okay, but didn’t live up to expectations.

To go on more of a tangent, I really thought these books would be impossible to turn into films, but the Villeneuve films are good so far!