Star Trek, when good, was the best and most important fictional TV show ever made.
Unfortunately it reached those heights on only a handful of occasions across all its series. THE DRUMHEAD seems more relevant with each passing day. DARMOK, THE MEASURE OF A MAN, and THE INNER LIGHT are triumphs of single episode TV.
Those aren't the best episodes of Trek and they're all TNG but they're the most important - for the large effects they've had on ethics, philosophy, AI and linguistics research and other topics alone but also how they stand as moral guidance to a humanist outlook.
Outside those episodes that are important, a and a few more that are just good TV, I can take it or leave it most of the time. When Trek is bad, it is grotesquely cringe inducingly bad