I would bump out "The Omega Glory" for this one. Kirk, Spock, and McCoy at the OK Corral. DeForest Kelly actualy played one of the Earp brothers in "Gonfight at the OK Corral".

The 3rd season was the worst. Did you know Gene had no involvement with Star Trek production during the last season. NBC screwed him on a time-slot deal. The guy driving the show in the 3rd season basicaly wanted "t*ts in space" and that is what he did. Read David Gerold's book "The Making of Star Trek" for the full story.