Ezio, especially in Revelations (though I may be biased, as that was my favorite game in the series by a significant margin). Awesome skills? Check. Years of experience? Check. Good outfit? Check. Beard? Check. Wit? Check. The only time I wasn't happy to see him on screen was at the end of Assasin's Creed II. That was, bar none, the stupidest, most poorly contrived thing I could have imagined him doing. Seriously, letting him go? That's it? It made NO SENSE. But beyond that, loved him.
Haythem and Edward were also very good, albeit for different reasons. The playable sections with Haythem were much more entertaining than later on in the game (when his attitude started to sour). Edward was great from the get-go and had very human failings that made him more seem the most real of the bunch, yet not so repulsively terrible a person that I didn't want to deal with him. It's a hard balance to strike and where Haythem eventually tipped off that balance, Edward stuck it pretty well, I think.
Ratonhnhaké:ton though, I couldn't stand. Part of this is personality, part of it is the setting. Early America is one of my least favorite periods of decently recorded history (at least for the region, maybe the rest of the world was more interesting at the time), and he doesn't have his father or grandfather's charm to overcome my dislike for the era. He's a little better when he was young (his taunt at Charles Lee still makes me grin at the audacity), but for the most part he just seems too uninteresting.
I have few strong opinions on Altair, although some of his later actions weren't bad. His final scene was one of the best examples I can think of for poignancy done correctly.
