That is my opinion of how badass the assassins are.
Ezio is the coolest because he championed the dual hidden blades, he has an awesome sense of humor, he's full of charm and wit, he's a ladies' man, and he kicks some serious papal ass.
Altair is awesome because he's the Original Gangsta Assassin, he's the one who is truest to the word "assassin", being middle-eastern and all, and he had a badass story.
Haythem is awesome because he is so suave. He reminds me of an old-timey James Bond, with that awesome accent, and the way he kills people is awesome too. He looks like he's dancing a ballet or something, but really he's ****ing people up with those hidden blades and that sword. Plus the opera scene was awesome, and every word of dialogue that comes out of Haythem's mouth is academy award-worthy.
Connor is my least favorite of the assassins in terms of story. In terms of gameplay, he's my favorite because I love that rope dart to death. Coolest weapon ever. But he has no sense of humor, and unlike Altair he doesn't seem to have a strong moral compass. He doesn't seem to care for anybody save for his village, he gets all huffy puffy at everybody, and he was a total ******* to the wise old black man. NEVER sass the wise old black man. That's rule number two, right after Nothing is True and Everything is Permitted.
For me it's Alta´r> Connor> Ezio. I didn't really like Ezio. His personality felt more comedic than anything, and coming off Alta´r, that's not what I felt an assassin should act like. As for Haythem *SPOILERS!!!!*
He's not even an assassin. He's a templar. So he should not even be in this list.
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The only thing I've read is his father was an Assassin... and Heytham joined the Templars at an early age. Others have said there is an email which confirms he betrayed the the Assassins, but I can't find it.