I think Saw would have to be his best. It was quite influential and the story is decent. The sequels on the other hand...

Insidious would have to be Wan's most creative but also his most uneven. The first half was genuinely scary (especially with the ghost screaming in the baby monitor) while the second half was goofy. I would like to see Chapter 2 but I hear that it's pretty much the same as the first.

The Conjuring would have to be his most technically accomplished but the only scary moment was when one of the daughters freaks out at the shadow behind the door. The exorcism was rather disappointing and I was more interested in the cursed items the exorcists kept than the rest of the story. Still a solid effort.