Robert De Niro has played many great criminals in his time, from Neil McCauley in 'Heat' and Vito Corleone in 'The Godfather: Part II', but his greatest statement to the genre was in 'The Untouchables' when he talks about "enthusiasm" while enthusiastically mauling someone's head with a baseball bat.
The only aspect of De Niro's portrayal of crime lord Al Capone that isn't convincing is that he's captured by Kevin Costner.