All boring.

brotherhood. It really fleshed out Ezio as a character and the gameplay improved so much from II.

I don't really know for sure. I really like ACII, Brotherhood, and III.

ACII was the one that made that giant leap forward technologically.
AC Brotherhood brought all AC II had with a better story, and a bigger spotlight on Desmond.
AC III was massive. It has the best animus storyline out of all of the games in my opinion. Haytham FTW! Also those Naval fights were amazing.

ACI is decent, but good lord is it repetitive. As for Revelations. It was at the point where traversing one city and some instance running dungeons was getting stale as hell. I know they're probably not going to do this, but I really wish they would go back to a 2-3 year dev cycle. The one year thing really hampers what they can do with AC.