Hard to say, but if you put a gun to my head, I might say Jeff Bridges. He can be funny, tough, he's got that great voice...I love Eastwood, but I don't really buy him as a three-dimensional character in most of his roles, more of a "type." I could say the say thing about Mifune, who would also make my top five list. But I really buy Jeff Bridges most of the time.
Sadly, he hasn't been in a lot of movies I've really liked, but I'm trying to seperate that from the actor for the sake of the exercise...