I forgot about Lee Marvin in his role in Cat Balloo. I'll never forget the part where he was drunk on his horse and leaning against the wall. The young indian said, "your eyes look terrible". To which he replied "You should see them from my side." too funny.
But I don't think you'll find any guys better than Selleck or Elliot in a cowboy movie (other than the legends of the west like Wayne and Eastwood).