I couldn't narrow it down enough so I decided to name 10 different actors instead. In no particular order:
Al Pacino as Tony Montana in Scarface and as Michael Corleone in The Godfather/The Godfather Part II/The Godfather Part III
Denzel Washington as Alonzo Harris in Training Day, as John Creasy in Man On Fire and as Rubin Carter in The Hurricane
Tom Hanks as Forrest Gump in Forrest Gump and as John Miller in Saving Private Ryan
Robert De Niro as Vito Corleone in The Godfather Part II, as Jake LaMotta in Raging Bull and as James Conway in Goodfellas
Sean William Scott as Steve Stifler in American Pie/American Pie 2/American Wedding/American Reunion
Will Ferrell as Brennan Huff in Step Brothers, as Frank Ricard in Old School and as Ron Burgundy in Anchorman
Will Smith as Chris Gardner in The Pursuit Of Happyness and Robert Neville in I Am Legend
Zach Galifianakis as Alan Garner in The Hangover/The Hangover Part II
Daniel Day-Lewis as Bill Cutting in Gangs Of New York
Samuel L. Jackson as Jules Winnifield in Pulp Fiction
I could name loads more but as that is 10, I will stop.