As Steve said, John Ford, the director, made John Wayne. His performances in all the John Ford movies, even the non-western "The Quiet Man," were very good. He was also good in a couple of other war films and westerns that were not directed by Ford. However, as John Wayne got older and, I guess, lazier, he started churning out 'formula' westerns that more closely resembled some of his 'B' movies from the '20's and '30's.
Clint Eastwood had a higher percentage of movies in which he played his character well, but most of them were playing approximately the same character. He is much more knowledgeable about film-making and what works on the big screen than Wayne was, or at least more disciplined.
I never considered John Wayne, Gary Cooper, Clint Eastwood, or Randolph Scott to be great actors, but they all captured their western characters pretty well. The guys who were in westerns who really were great actors were the ones who were in all different kinds of movies, as character actors, or a few in lead roles like Burt Lancaster, Jimmy Stewart, Montgomery Clift, Karl Malden, Kirk Douglas, Glen Ford, Robert Taylor, Gregory Peck, or maybe even Alan Ladd.