I love a good gangster flick - mainly due to the really psychotic, sociopathic monsters that are portrayed. For every manic, psychopathic Tommy De Vito there is a cunning, calculating Michael Corleone, and a wannabe like Travis Bickle showing all the traits.
My favourite would have to be Michael Corleone - who in one swift movement killed all of the Corleone family's rivals. He is turned more and more into a loner and by the third (abeit worst) Godfather film, he is insane. Tommy De Vito, Jimmy Conway, Al Capone and of course Tony Montana are also good portrayals, as well as Carlito Brigante.