both of them were very bad ass but in different ways. altair in the way that he knows his mission and he will do what he needs to do to make sure he finishes the job. ezio in how he chose who to kill based on personal vengeance (rodrigo borgia) or to help people. in a stealth fight it would be very hard to choose but i would say altair, in a sword fight again, hard choice but id say ezio. in AC 2 i dont recal a lot of stealth kill missions, when u kill vieri de pazzi u openly invade the city and kill him in a sword fight, thats just one example but its late and im tired :P