The best Assassin's Creeds were any game involving the Borgia as the enemies. Those are awesome villains right there. Assassin's Creed: Brotherhood is probably the best game of the entire series. It did whitewash the Medici a bit, but eh, understandable in the context. The game was fairly open world and allowed a fair amount of creativity in killing enemies. I always liked killing a roof top guard, dropping the body and killing the folks that came to check out the mystery corpse. Except for the time I accidentally dropped the corpse on the guard, killing him before I could stab, poison or shoot said guard. Whoops. It really had its moments. Toss a smoke/stun bomb into a pile of guards, walk through, kill ONE guy, and be gone before anyone knew what the heck happened. Or do the slow straight Epic-Bad*** walk as your minions kill friggin everyone? Combined with an epic plot? Good times, good times.
3 was alright, but I was disappointed by the ending of the series. Awesome game mechanics, plot was mostly pretty good, but... They did drop a lot of the more creative kill methods. I didn't like climbing trees, it sucked. I did like stabbing hundreds of redcoats to death. Very satisfying. They dropped the ball in the end, imho. Still worth buying.
