Michael Rennie in The John Cameron Story. He did not fare well because he did not have Gort with him.
Dan Duryea in many episodes. They should have made him a regular.
Raymond Massey as an Aztec king reigning over a hidden city somewhere in the Southwest.
Yes, and McCullough talks him out of sacrificing his daughter, and his subjects kill him. 
There was an episode of Wild, Wild West with an Aztec city hidden somewhere in the Southwest, except that one had a hot queen, IIRC, and (for a change) no one there spoke English. I always wondered if they were speaking real Aztec instead. 
Unlike most native American languages, Nahuatl, the language of the Aztecs, actually spread in influence during and after European colonization. At the time of these events, it would have been fairly easy to find interpreters. In the present day, it has approximately one and a half million native speakers, more than any other North American language (Navajo comes in second with about a tenth that number).
Lee Marvin as the evil new wagon master in “The Christopher Hale Story”
And we never discover what happened to Seth Adams.