I think Altair
I think it's Ezio because even though Altair was the first Assassin in the series Ezio has more moves and is also just shown to be a better Assassin overall. However, you could also argue that Altair was better as Ezio's best equipment when he is first introduced is Altair's sword etc. and technically speaking even right till the end Ezio followed Altair's path so I guess they are equal in some aspect. They were both great Assassins. They were both great people. They both experienced hardship when young and most of all, they were both great characters and are what made the Assassins Creed series so great today.
So I guess I take it back, they are equal.