I'd toss in Jimmy Stewart and a few others as well. ;o)
Actually, the Duke was a very bad actor, but made up for it with screen presence and a deep understanding of what makes a movie entertaining.
As of yesterday, I'll have to add Henry Fonda to that. Althogh I'm a major western fan, I had never gotten around to watching the movie "Once upon a time in the West." I saw the DVD at target for 9 bucks so I got it and watched it yesterday. Henry Fonda was awesome as the bad guy. I have never seen Henry as a bad guy so there was some cognitive dissonance to get over, but he was really great. I guess you can give a great actor any role and he/she can make it theirs. Trouble is, all we have now are movie stars, very few good actors left in Hollywood.