I would have to say the most impressively ambitious and consistant is lord of the rings.
Harry potter would be next in its consistancy of delivering a decent translation of the books, as well as advancing the character development. It has also been interesting seeing the children grow into their own as actors. The first two movies showcased poor to better than average child acting. I believe the first two were saved by the directing, but the next three installments featured great improvements in the casts acting.
Lastely i would say the matrix was the most bold and original. Even though droves of people were dissapointed in the films sequal. You have to appreciate that the brothers told their story and not a story influenced by a studio.