I don't know about it being the worst, but it was certainly poorly written, poorly executed, and confusing as hell. As with all of TOS, there are elements of it that are good. For example, I loved Lazarus's little ship, a nice retro-throwback to the golden age of sci-fi. It also had a unique from-behind f/x shot of the Enterprise firing its phasers at the ship on the planet surface below.
It was still better than most of the Voyager episodes that I've seen.