I voted for DS9.
IMO:
TOS:
The Original Series shouldn't be in the same category with the other shows. Apples and oranges, as far as I'm concerned. That said, I'd still rather watch "Trials and Tribble-a-tions" than "The Trouble With Tribbles."
TNG:
This was a good, solid show. I loved most of the characters - even the Crushers and Troi eventually grew on me. (Doc Crusher taking command of the Enterprise in "Descent" was excellent, as was Troi's bridge officer test.)
The only thing that really bugged me about it was that it was too formula, and not enough arc. I loved episodes that actually connected to each other.
DS9:
This had everything I liked about TNG, except specific characters, but it also had more unusual stories, and it had an overall arc. Plus, the show itself was gritty - I really liked seeing the Trek universe from someplace *other* than the bridge of a Starfleet flagship.
Plus, it had my favorite episode of all time: "In the Pale Moonlight." It's the one where Sisko allowed a Romulan ambassador to be assassinated, in order to turn the war around? One of the finest hours of TV I've ever seen.
So, DS9 wins.
(I did have a few complaints about the show: the prophets started out really cool, but ended up lame in the final season, and Gul Dukat turned from a complex, layered villain to a comic book freak...but the show accomplished so many other things that those are forgivable flaws.)
Voyager:
This show was garbage. It had a few really good episodes. It did. I actually liked the adventures of Captain Proton (sue me

), and I really liked "Timeless."
But overall, it was just garbage. Among other flaws:
- They ruined the Borg.

- Captain Kirkina - I mean, "Janeway," was criminally insane. (This is what happens when a loose cannon like that doesn't let off steam with green alien space babes - the frustrations makes them blow *everything* up, instead of just Klingons.

)
- They killed Kes! (Those bastards!)
- It didn't just lack story arc. It lacked continuity entirely.
I could go on, but...meh.
Enterprise:
I've watched 2 whole episodes. (Sat down for 4, but I hit reruns twice. Bad omen.

)
It's okay. Definitely better than Voyager - from what little I've seen, the show is at least internally consistent. But it really just felt like a rehash of other shows, so it couldn't hold my interest. Same old aliens (even Ferengi), same old plots, same old bridge crew.
I wish they'd done something creative, this time around.