Ray Patriarca Sr - IMO Ray Sr is easily one of the most successful mob bosses of all time. the whole "Father Fagin" thing where he fixed his way out of the joint is legendary too. there was no one close to as powerful as him in new england which i think is a big reason things got so ugly when he passed.

Tony Accardo - pretty much all the reasons listed. no way you can't respect his reign.

i gotta throw an honorable mention to sonny franzese and larry zannino, two real old school hitters who never ratted (or in sonny's case, never will).