I've always said that Ron Tracey was the most evil enemy encountered in the Original Series. He not only committed mass murder and violated his most sacred oath as a Starship Captain, but he tried to bring the end down on Kirk and the rest of the Enterprise crew. He killed the security officer in cold blood, almost killed Spock, was cold about the death of his crew, and would have slaughtered Kirk as well. Just a ruthless, selfish, evil, man. A lot of the other villains were either comic bookish / mustache twirling or were misunderstod / shades of gray types. Tracey was just evil. And, the fact that this is Kirk's peer, the Captain of a Constitution-class starship...that makes it even scarier.