sorry to bring up old threads, but i'm new here and this one is still close to the top of the page. so here's my .02$
for the most part, none of the old actors protrayed real cowboys especially well, until Ben Johnson and Slim Pickens. because they actually were. never thought John Wayne was the best although i have read many times that he copied his walk, talk, mannerisms, everything from Yakima Canutt who was a real cowboy, world champion bronc rider, and early stuntman. I like Sam Elliott, but he doesnt really come off as a real cowboy. Tom Sellick is better, and his more recent movies have been more historically correct than any others, but still not exactly correct in every instance. as for really being beleivable cowboys (not just gunfighters, westerners, fronteirsmen, etc., but REAL COWBOYS) in no particular order, here is my short list of favorites. not all of them rode especially well, but had the speech, mannerisms etc., down pat
Tommy Lee Jones
Robert Duvall
Richard Farnsworth
Ben Johnson
Wilford Brimley
Slim Pickens
Lee Marvin was really good in the origional Monte Walsh as were Glen Ford and Henry Fonda in The Rounders
but my all time favorites are Robert Duvall in Lonesome Dove, and Tommy Lee Jones in The Three Burials of Melquiades Estrada, and The Good Ole Boys. those 2 guys in those films played real cowboys better than anyone else ever did. and Ben Johnson gets the nod too, because he really was a real cowboy, more so than any other actor, modern or not.