Stifler too funny in this movie - without him, there is no movie
I know Stifler was really juvenile in this movie, but he had some classic and funny lines. I would have to say that for Sean William Scott I like him best in American Wedding and the Rundown (one of my favorites). This guy has a face made for playing a douchebag - I don't mean that as an insult. I think he made the movie interesting. Otherwise, there would have been few opportunities for gags and other funny parts.
You have Finch who is pretentious and tries to act "sophisticated"
Jim who is neurotic, anxious, and uncomfortable all the time
the dad who gives hilarious advice
the bride who is just a crazy little nympho
the uptight parents of the bride
Kevin who really has nothing to do at all
and Stifler who actually drives the plot.
Otherwise it would have been a boring movie about two people getting married.