My favorite is the bartender in the attempted hit on Pentangelli. He's such a mensch letting the guys use his place to whack Frankie Five Angels. He wipes that bar beautifully, like a TRUE professional. And we can really sympathize with his begging Carmine not to blow a cop away in his establishment.
Seriously though...I don't really have any favorite characters/actors. I enjoy each along with the story as as they come along. All do their part (big or small) in making these films the classics they are.
Apple