Assassin's Creed - Awesome in the beginning exploring and attacking guards and such, really slows down towards the middle and gets really repetitive, picks up near the end.
Assassin's Creed 2 - Pretty straight forward start, not as exciting as the first one. Variety along the way and it has a pretty consistent pace throughout the entire game, the end will either keep you going or push you away.
Brotherhood/Revelations - Just spin-offs which are very similar to AC 2 with some stuff added here and there.
Assassin's Creed 3 - Probably the most interesting characters in the series, and has a nice pace and interest up until the end where it sort of just drops.