I think it’s appropriate to mention Bob Feller today. My father ybl”a thinks he was the greatest pitcher of all time, and he saw many of the great ones.
As to Walter Johnson, it’s hard to argue. This of course brings up the eternal problem of comparing greats from different time eras. Mantle is not a good example, he struck out against everyone. Koufax said himself that his toughest out was Hank Aaron, who batted around .320 career against him, I think the only player who batted over .300 against Koufax.