I am a little late to the discussion, but my most intriguing guys would be:

1. Chin
2. Lansky
3. Accardo
4. Carlo
5. Carlos Marcello

I would love the opportunity to "interview" these guys. All were great leaders.

Marcello makes the list just so I can find out if he and LCN actually did have a role in clipping JFK.

All the psychopaths previously mentioned (Casso, Demeo, Pitera, Scarpa) are a dime a dozen.

Leaders who spent most of their career as boss with limited jail time are rare.

IMO, that makes them more intriguing.