Altair was the perfect assassin: he was very efficient and precise and he was a master at what he did (literally.) If I wanted a target killed without fail I'd pick him.
Ezio was a Jack of all trades: he was extremely talented with general melee weapons and was pretty handy at brawling with his fists having best the hell out of quite a few of his rivals in 2 and brotherhood. He was good at adapting his tactics too and embracing new and unusual weapons/techniques such as the hidden gun and the grappling hook from revelations. I think Ezio would be the best for an uncertain situation where you might need to apply any given number of different techniques.
Connor seemed to me like more of a warrior than an assassin personally, his combos are beyond lethal, also the brutality of the kill animations and his choice of weapons like the tomahawk and spike-rope make him ideal for open combat. Even his outfit looks like that of a soldier's with the period buttoned coat and everything. I find I'm having the most fun with Connor when I snare a Redcoat officer with the Spike-rope and noose him up from a tree in front of his own patrol, I then fire my pistol into the drummer before he can run of to snitch to the other guards before whipping my tomahawk out and burying it into a brutes head. Within about ten seconds a dozen redcoats are lying dead before they can even react and the woods are quiet once more. Connor is a powerhouse, pure strength with stealth elements when required for things like ambushes/assassinations.
"I used to rescue presidents, then I took an arrow in the knee...and still rescued the president." -Snake Plissken