"Arena" is a minor classic, essential mostly for the iconic fight between Kirk and the Gorn, one of the few truly alien-looking aliens from the series. The Gorn; you know, the man-sized lizard (obviously made of rubber...with a zipper an' everything!).
It's one of the key memories any fan has of the series, actually. The episode around it was duplicated a number of times, probably most successfully by Next Generation's "Darmok." It's actually a little like the classic "god box" template, beings of greater power than mere humanity interfering and judging and things of that nature. So there's that going on, and that's how and why Kirk ends up fighting the Gorn.
The image of the Gorn was so iconic that even though these particular aliens didn't show up again until Enterprise ("In a Mirror Darkly, Part 2"), they were among the most famous aliens in the whole franchise. Pop culture remembered it well enough, too:
franchise * series * essential * character