Probably the mobster who intrigues me the most is Anthony Casso like a lot of people here, so there's no point in explaining it too much.
I am also intrigued by the DeMeo crew, but the Gemini Twins (Anthony Senter and Joe Testa) not Roy DeMeo so much. Those guys were up there with the most brutal and efficient hit squads in mafia history, and those two did a lot of the work. Senter was a tough son of a bitch; fought pretty hard growing up and he even ended up killing Roy DeMeo himself...Also, the fact that his family was from the Northern Italian/semi-German speaking area interests me. Pretty rare.
I also think Nino Gaggi was a very smooth, effective operator who did things the old-school way that earned him a lot of respect.