TNG. The first season is watchable, and later seasons are some of the best television ever made.

I don't recommend watching the entirety of TOS, since most episodes are fairly weak. But it's worthwhile to look up a "top ten best episodes of Star Trek The Original Series" and watch a handful of those that sound interesting. They will also give you the background you need if you decide to watch the Stark Trek movies, which are a mixed bag, but enjoyable.

DS9 is the next best series, and a lot of it is on par with TNG. But it's less episodic, and not a great entry point in the series.

VOY and ENT are a mixed bag. There's some genuinely great stuff in here, but I don't think you should seek them out until you get really into Star Trek.

New trek stuff is not worth watching, except for Lower Decks, which is a comedy, but also good Sci Fi.

Galaxy Quest and The Orville are comedy parodies of Star Trek that also happen to be surprisingly earnest Sci Fi. Worth watching after you finish TNG. Kind of a running theme of Star Trek is that the comedy parodies of it are great.