Mines a toss-up between Ezio and Altair. Chances are I would have liked Altair better, but I had 3 games to get to know Ezio where as only like, one and a half to get to know Altair. Granted the Altair bits in Revelations were pretty interesting, I just liked Ezio better. Specifically, Ezio in Brotherhood would be my favorite.
As for Connor...I haven't beaten 3 just yet (just took down my second main target), but I'm having trouble getting over the fact that he's an impertanent ass-hat. Granted, I accept that he's SUPPOSED to be an impertanent ass-hat as part of his character. A native american boy that sees his family get slaughtered (not a spoiler since that tid-bit is revealed in the trailer for the game

), it's understandable that he'd be walking around with a chip on his shoulder. I just wish he respected his elders a bit more, Achilles in particular. This man gave him equipment and training and a path to his destiny, but due to his impatience Connor can do nothing but ***** about how things are moving too slow. He doesn't want to help anyone or do anything that won't also help his people, and literally has to be convinced to go a single mission further by having someone explain to him that by doing this he'll also be protecting his people and their land.
Again, I accept that these things are supposed to be there, that him being brash, abrasive, and impatient are all part of his character. Doesn't mean I have to like it, though.
I like Ezio because to me, he epitomized what it meant to be a leader amongst assassins. A wealthy noble by day, stalking the shadows and roof-tops by night. He was smooth and charismatic, qualities that I think a master assassin needs, and I just don't see those qualities in Connor or Altair.
And on a side note: Revelations fucking sucked. Yeah, I said it.
