I'm with Maxpower for my favorite Spaghetti Era cowboy.
Franco Nero for sure!
For classic era, 30s, 40s, and 50s, John Wayne
Otherwise,
Eli Wallach is pretty high on my list
Clint Eastwood, even if he didnt really act very well, he got the image right. And when he directed and stared in movies, they didnt turn out too bad either (I love The Outlaw Josey Wales and Unforgiven, though Fistfull of Dollars is probably the best movie he's in, and I love Good Bad & Ugly)
Lee Van Cleef, I am a bit less a fan (I'd take Eastwood over Van Cleef) But I do enjoy his performances...