Having completed all three games back to back to back, here's my view in a short blurb for each.
Assassin's Creed: The best thing about this game was that it gave us Batman: Arkham Aslyum and Assassin's Creed 2. Altair is an unlikable jerkass and the fact he becomes less of one doesn't really improve the fact you start the game as a psychotic murderer and terrorist. Add in the fact that you have to do a LOT of horseback riding to get from place to place.
Assassin's Creed 2: This game is pure awesome. I cannot think of a single thing wrong with it, at all. It's just great fun from start to finish. I recommend it to everyone. Ironically, if there is a complaint, it's that it makes AC even less fun because you see where they could have gone with it.
Assassin's Creed: Brotherhood: Suffers from Dragon Age: Awakenings syndrome. Basically, it seems obviously rushed out despite the beautiful attention to detail setting wise. It plays a lot like Grand Theft Auto of all things with horses replacing cars and amps up the swordplay 900%. Which wouldn't be bad if not for the fact that swordplay is the least fun part of the game. A lot of the game is also buying shops like in Saints Row.
So I'm eager for Revelations but oddly am not sure if I'll continue the series since I'm here mostly for Ezio and this is his last game.