1. DS9 best characters, best storyline, most interesting setting (all imho of course) This series was really the trek series that made its characters real people instead of just having them be players in the weekly drama. The promenade set was BEAUTIFUL and characters like Kira and Quark are among the most three dimensional ones Star Trek ever had.
Also even with its darker tone and war storyline it never abandoned the trademark Star Trek optimism.
2. TNG The start is weak and by the end the episodic formula got stale and turned into "what wacky, devastating crisis are we gonna face today" but the middle was delicious like the filling of an oreo cookie. Also love the atmosphere and the characters. Season 3 and 4 have the best storylines imho. Several important improvements and step ups from TOS (steps towards an ensemble cast, better developed characters, better production values) but there would still have been a lot of room for improvement in all areas.
3. Voyager So much potential....so squandered also characters that (in my eyes) were either bland, boring, underdeveloped or plain unlikable. Again the strictly episodic formula, no acknowledgement of their situation (most episodes might as well have taken place on TNG) There are a number of really good episodes, but most of it is jsut so boring.
4. Enterprise on pure account of Reed, the "Vulcans without logic" and the Andorians. Aside from that? Fairly bland characters, the ridiculous "Temporal Cold War" storyline rehashing of old ground that was way to 24 century for the timeframe of the series (Borg? Really?) I will however say that the uniforms looked kind of awesome.
5. TOS to me it was not the first Trek I saw so I have no nostalgic feelings attached to it, most of the characters to me are like the Voyager ones either underdeveloped, boring or unlikable (imo) that damn soft focus every single female guest star gets, all the conceptual, incredibly powerful villains and the numerous episodes set on planets that are just like an historical period from Earth.