I'd imagine that a mobster does not need to be highly intelligent, but clever in order to survive. Also, he has to have some kind of strategic thinking, I guess.

AJ is just outright stupid and weak, he would have been killed quite fast. Weird as it may sound, Meadow would be a good choice.

Un giorno, e non arrivi mai quel giorno, ti chiederò di ricambiarmi il servizio, fino ad allora consideralo un regalo per le nozze di mia figlia.
(Don Corleone)

Once you're into this family, there's no getting out.