Altair was way too cocky, and there was no story with his life. He was an assassin, big whoop.
Ezio, on the other hand was the son of a wealthy banker/ undercover assassin, and took on the role of his father, which makes for a much better story.
Plus, Ezio dressed better!

Altair
was cocky and arrogant at the beginning. He is later humbled as
Spoiler he has to go through the steps of becoming an assassin once more after his botched mission.
Ezio is some punk kid who was thrust into becoming an assassin and should not have any other skills than street fighting. His sword skills at the beginning are a mystery and really does not make any sense as he can pull off some amazing attacks and aerial maneuvers as he moves from rooftop to rooftop despite being a novice (plothole?)
It's only later, 2 years later, when he returns to Florence two years after training with his uncle that it makes sense that he has some skill. Even then, he would be no match for Altair who had trained his whole life as an assassin and had been in many real fights that would have made him a battle hardened warrior.
So in terms of skill, realistically Altair is way better. I also like his cold attitude that many feel is underacting (from reading the thread) but I think he plays out a cold killer who is used to battle.
My vote goes to Altair