In rank of best to worst:
American Pie
American Wedding
American Pie 2
If you judge it objectively, I'd say Wedding comes out on top but in 1999, this movie seemed to come out of left field. Nobody I knew (I was 15 at the time) thought it was going to be good or even decent. Since at the time, teen comedies were groaners rather than knee slappers. For the effect, AP is the best.
AP2 sucked. Very few funny moments put in there although the script was better than AW or even the first. It was well written but not hilarious. In fact, come to think of it, I didn't laugh once. My friends and I went to see this first week in 2001. I remember how packed it was. We must have waited 45 minutes for tickets and we almost were at the cutoff point. Also, another disappointment was that the nude scene in AP2 was much worse than in the first.
AW gets a lot of undeserved attacks. I think the effect from the first movie was gone and AP2 was one of those movies everyone thought was hilarious walking into the theater but walked out pissed. Went to see it maybe a month after it came out but still had a bit of people there watching it. Too many good moments.
I hope its finished off because an AP4 would probably be a box office failure. I don't think AW fetched any decent numbers either but AP1 and 2 were pretty big.