I think Deep Space Nine is the best Trek series by a lot. It's the first to establish a serial storyline with the Dominion War and IMO has the best characters.
Also has two of Trek's best villains in Gul Dukat and Kai Winn.
Man, only 5 of us even ranked it. It will always be my #2 Star Trek series, but that's not a slight in any way.
"In The Pale Moonlight" is one of the best episodes of t.v. ever. There were a ton of other great episodes as well. The show maintained the tradition of great sci-fi expositions of current day issues cast through the lens of other times/places, while also moving farther away from the episodal format.