I think I’d struggle to come up with such a list in a way that it’d be a rewarding viewing experience.
Something to keep in mind is that almost all of Star Trek from before the streaming era was made with the assumption that people would tune in and not necessarily have seen any previous episodes. Especially with syndicated reruns, there was no real guarantee of even seeing the episodes in order. At most you would be expected to miss some details if you caught the second half of a two-parter, and those had recaps at the start.
Star Trek only really started to do big plot arcs with DS9 and even then it was still very episodic. The earliest they had a modern BSG-style serial plot was season 3 of Enterprise. So there’s really no need to watch “must-watch” episodes. The episodes were made with the intention that you could watch each one with minimal context and still have a good time.
I don’t like zombie movies. I watch Star Trek because I like Star Trek, not zombie movies. For an episode to score the highest reviews, it needs to appeal to a wide range of fans, so I would not expect a take on a different genre to score as well as a traditional Star Trek episode.