I don't know about absolute worst but I've always found the episode Omega Glory to be pretty embarrassing.
I always loved "The Omega Glory".
"We killed thousands and they still came!"
I think it would make the basis of a kickass movie.
Most of the episode was fine, and I always thought that it was a missed opportunity by the spin off shows to explain why TOS found so many parallel earth civilizations, like Omega IV (Enterprise sort of touches on it in North Star). Captain Tracey made an excellent villain. The only thing that makes me cringe is Kirk reciting the constitution.