While perhaps he wasn't the most violent mobster, he brought good business sense to organized crime, which in the long run is even scarier that just killing people.

Did some strong arm work back in the Five Points of New York, before Johnny Torrio brought him out to Chicago.  He was almost certainly the killer of Big Jim Colosimo, which cleared the way for Torrio to move heavily into rum-running. 

Capone enjoyed the spotlight, and loved being seen at operas, ball games, and the like, as well as donating prominently to various charities.

While he did most of his killing at arms length after Colosimo, he was certainly capable of getting his hands dirty. 

While he developed a reputation for violence later in life (as syphalis started eating away at his brain) Young Alphonso was as charming and friendly as a hitman could be. He would rather cut a deal than rub out the competition, and was not a sociopath like many of the characters on this list, despite his reputation (mostly garnered by sensationalist Depression-era Hollywood movies).