I don't mean worst, just what seems to you to be the strangest or most far out. For me it was always "The Empath". This was the last episode I saw from TOS, as I had missed it during the original airing, and did not catch it in a re-run until about 15 years later, so it was kind of a "lost episode" for me. The whole look of the episode is strange, from the stark sets to the final fade out, where the Vians and Gem seem to just fade away into the darkness. The whole thing had an "Outer Limits" quality to it, like a long dream sequence. I always saw this as the strangest ep of TOS.