Spock's Brain was bad, but I think the worst one I ever saw was the one where Kirk got body snatched by the female scientist. Remember this was during the 60's. That episode was so bad it set the women's lib movement back decades and by some accounts was directly responsioble for the failure of the passage of the ERA amendment.
Watching Bill Shatner pretending to be a woman was just disturbing. [shudder]