why is mel gibson on that list? isn't he aussie? from that list i would say Brando. but you forgot great actors such as jack and depp. there is one actor who imo betas them all although he isn't america: daniel day-lewis.
James Stewart...because he played a greater variety of characters over a longer number of years than almost any other male movie star.
Al pacino. His performance in the scent of a woman is the most honest performance I have ever seen in a movie.
If you know how other actors have answered this question over the past 40-60 years, the answers most given by other actors are Spencer Tracy, Edward G Robinson, Marlon Brando, Robert De Niro & Robert Duvall.