#55: The Gamesters Of Triskelion
In case you forgot: Uhura, Kirk, and Chekov are abducted by brains-in-cages that collect specimens for their own gladiatorial amusement.
Putting gladiator-style combat into a credible sci-fi scenario is more credible of an idea than just a parallel Earth stuck in Roman times. Not that I'm naming any episode in particular...
I think, in a lot of ways, this episode is pure Star Trek stereotype. You've got dopey fight choreography, aliens who have ~evolved beyond the need for their brains~, Kirk romancing a green-skinned (or green-haired, in this case) space babe, and Chekov's accent. But beyond the stereotypical trappings there's not a whole lot interesting or unusual going on here. It's certainly not an episode that merits rewatching.