The way I see it is:
Altair: Wisest and most dedicated Assassin. Lived and died an Assassin, Ezio wasn't born an Assassin and he retired before he died. Also he probably had the biggest impact on the Assassin's Order thus making him the most legendary Assassin. He changed the way all Assassin's work! I mean he brought love into the Assassin's.
Ezio: Most charasmatic and passionate Assassin. He was a lot more emotional than Altair. Probably the best Mentor as well. I mean many Assassin's didn't like Altair as Mentor. He struggled in that department. Not until he was an old man could he get everybody on his side.
Connor: From the little we've seen. It definetly seems like he's gonna be the most skilled Assassin. Think about it in that CGI trailer he took out ALL of those British soldiers who had freakin GUNS! He is definetly facing the toughest, most well trained and best equiped Templars of the three of 'em. And let's all assume that he is probably going to be the key person who wins the war for the Colonists meaning he may be the first Assassin to help win a large scale war that shapes an entire country.
So I'm gonna take a chance and say will be the best Assassin. Desmond of course will be the best Assassin, BUT only by cheating sense using the Animus. lol And of course he's probably gonna save the entire world. Which just can't be beat.
Ok, I think if it came down to a street brawl, it would be Ezio.
If it came down to a sword / dagger fight, it would be Altair.
And so far from what I've seen, if it came down to an arrow / gun fight, it would be Connor.
WTF? yopu do know that conner is statitically the biggest and strongest assassin right? altari and ezio barely reached 5'10(which was tall... for there time period) Conner is a 6'0 plus. the devolpers even said it. conner is more muscular thus takes more effort to be as agile as ezio and altair... altari and ezio are the same ppl coding wise. The best gunsman is conner, the most agile is conner to since he overcompensates because of his size and STILL runs faster than ezio and altari. ezio is not stronger than conner BY FAR. have you played with conner alot yet? the freak of nature picks fools up and slams them on there head effortlessly.
Ezio= leader= proof: trained up to nine assassins at once
Altari= An True Assassin: trained as an assassin at birth
Coner= a killing machine, that simply refuses to be so.=proof: have you seen this guy fight? nuff said.