Oh dang I'm late to this thread. Hard to top many of the comments here I feel did a great job of summarizing things. So I'll add a few of my thoughts. I believe the tv series can be ranked as follows from best to not so best:
1. TNG. I firmly believe TNG is not only the best Star Trek, but one of the best TV series of all time. On a similar note, "The Best of Both Worlds Part I and II" is, imho, the two best episodes of tv possibly ever. Not only is it just generally awesome, but it introduces likely the best Star Trek villain, involves Q, and is a nice wink at the audience about the dangers of arrogance. It showed that even someone like Picard could fall victim to his own ego.
2. DS9. Fantastic series. Other than Picard, Sisko is my favorite captain. As others have mentioned, the plot can be a bit hard to follow especially at first, and there's a lot of political intrigue to follow and keep up with. Notable episode: "In the Pale Moonlight", where Avery Brooks really shows off his acting chops. It's one of the best Star Trek episodes period. It has the best final monologue/ending.
3. Original Series. I didn't grow up with TOS like some here did - I grew up with TNG so I'm biased. Won't touch too much on this one since others did so very well and showed the OP the top episodes to catch. City on the edge of forever is, like the best of both worlds, one of the best episodes of tv period.
4. Voyager. A lot of hate in this thread for Voyager, but I think it's good. I actually really liked Janeway as a captain. And yes, 7 of 9 was a nice perk

I haven't watched Voyager in over a decade, so I can't remember any notable episodes to name.
5. Enterprise. Don't get me wrong, the show is not BAD per se, but the concept fell flat. It's a prequel to TOS. Worth checking out when or if you get through everything else.
6. Discovery. HORRIBLE, contrived, woke political garbage. They screwed up the Klingons and said publicly the Klingons are now a metaphor for Trump supporters. They screwed up so many things even in the very first episode. Threw out any ideas of honor or ethics for captains. And don't get me started on the captain. An in your voice, BLM woman whose in your face about her "blackness" just for the sake of SJW cred. Don't watch this unless you want to get pissed off. Or if you're a liberal who likes that crap.
Picard - haven't watched. Based on a lot of the comments here sounds like I'm not missing much.
Lower decks is a new one I haven't seen either. There's also a "Star Trek: the animated series" that came out in the 80s I think.
CNN is an enemy of the state and should be treated as such.