As much as I love the Narnia movies, I have to say that I find the Potter movies better on the whole. The books, of both series, on the other hand, are just about dead even in terms of quality, IMO.
As for the Potter movies, my favorites are Chamber of Secrets, Prisoner of Azkaban, and Deathly Hallows I. The first two really set a tone for the films that I enjoyed and appreciated. Starting with PoA, that tone was altered. It wasn't so noticeable in the third movie, but by the time Half-Blood Prince came out, I could really see that this was a different atmosphere. As a result, as far as I'm concerned, the films and the books are in two completely separate continuities. Deathly Hallows I was a greatly welcomed step in the right direction.