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 do love a sequel years later out of nowhere! Ship in a Bottle is also a good one for that.