I am going to commit HERESY and say....JOHN WAYNE.
His early films were cheap and can be overlooked. Beginning with STAGECOACH he got good roles clear through the mid 1960s His JOHN FORD movies are great! Then with MCLINTOCK he fell into a rut and all his characters were the same. Just change the shirt from a faded red one to a faded blue one.
His last good role was in THE SHOOTIST.
And yes, I have seen all his late 1960s and ‘70s movies at the theater.
Wayne was consistently typecast, to the point where entire movies were written around his character. Take him out of that and give him a non-cowboy role, say, that of Genghis Khan in The Conqueror, and you'll see just...uh...how deep his...uh, well, you'll see what a versatile actor he...uh...
Never mind.