Anthony Casso was a real psycho and he had brains. That's a dangerous combo. Galante was feared but not feared enough to avoid getting clipped. Guys like Anastasia and Genovese were no picnic either. Toto Riina would slaughter babies... Most feared, I'd say The Chin for the simple fact that he tried to kill Costello and nobody dared touch him when he was the boss of the Genovese.