Altair. {WARNING POSSIBLE SPOILERS}He's been trained from birth, HE gave up a finger to use the hidden blade, beginning of his career he's basically swatting aside entire armies with a sword and a few knives(yup, he doesn't need guns, bombs, swords, knives, hammers,... to get the job done), killing templars left right and center(highly mobile roof climbing chainmail wearing templars at that), then going back and crushing adversaries with exactly the same training as himself, kills a pretty irritating teleporting Grand Master, and then goes on to expand the Assassin order out of the Middle East all the while creating awesome little gadgets. The best items Ezio gets are things that were designed or made by Altair : armour, sword, twin hidden blades,...
And all this ALONE.
Whereas Ezio is just a whiny ponce who takes years to grow some stubble and some balls, and has to pay prozzies to get the job done. He may also be a bit slow in the head. I mean, in the first Assassin's Creed, Altair's costume might not have been the most discreet either, but at least there were SOME other people who wore similar stuff, so he could reasonably blend in. What's Ezio's excuse?