I think Fannucci had the most character-appropriate accessory, his whole waredrobe was almost as important to his character's image as his actions were.

I like young Clemenza's, the bowler is just right for his character as well and contrasts just enough with his posh gears to make him just enough of a hood for us to pick up on.

Mike's hat does make him the most bad-ass of the whole bunch, I think, when he walks into the house with Carlo, with that hat, I was like, WHAAAT?, he looks so fucking bad ass.

Those are three that really stick out for me.