I’m going to blashpheme here and say Jack Nicholson. He always plays himself. The Departed was good, but again the bad accent ruined it. He’s clearly a fascinating guy on screen, and worthy of success, but I don’t see much “range”. I just see Jack Nicholson. The best actors are those that you forget that they are playing a role. Dustin Hoffman is a good example. You believed he was the savant in Rain Man. Daniel Day Lewis is another. He prepared for “There Will be Blood” for a year, getting the accent just right. He listened to recordings of speech patterns of old prospectors, and ended up sounding like John Huston. Believable in the role. Bad accents are like fingernails on a chalkboard.
Stars have always played themselves. Jimmy Stewart, John Wayne, Bogart. Actors are usually in supporting roles, or as so-called character actors, who also, more often than not, play themselves, or play the same role over and over again.
True actors are people with theatrical training which many of the stars lack. You see those in European films.