Actors who can only play themselves.

Nic Cage
Keanu Reeves

Vince Vaughn
Will Ferrell
Christopher Walken

Harrison Ford

Johnny Depp

But in his, and other "good" actors listed in the thread, defense, 95% of actors play themselves or a slight variation of it. Most people want to see Tom Hanks, they don't want to see Tom Hanks the crazy, psycho, child molester. They go to movies to see Tom Hanks, Tom Cruise, Keanu Reeves, etc.

Heck, I think that if Tom Hanks was never an actor, and was mentally challenged, he'd probably be a lot like Forrest Gump IRL.

Hanks, Pacino, Denzel, Depp, Crowe, and on and on. A lot of "A" list stars fall into the category because they are A-listers, and people want to see "them" not the character.

Usually, actors that bump that trend are middle of the road in terms of popularity (or even foreign for us), and can risk being a chameleon in a role: Don Cheadle, Jack Nicholson (in his early years), Gary Oldman, Leo DiCaprio (I would say he has recently entered the great actor list, bucking the middle of the road trend a little), Brando, Toshiro Mifune, Kinski, etc.

I love Jimmy Stewart, I think he is the actor that has embodied American cinema like no other. I call him the greatest American actor for his influence on the craft and on film (and even his dedication to military service, family life, etc.) The man is a legend, and will always be. With that said Gary Oldman is a much better actor overall.

Wil Smith