Interesting thread - I suddenly realise how much I actually like all of Trek! I have really enjoyed it all - but since this is a debate here I go:
In reverse order:
5. TNG (controversial!) - grateful to it for starting the franchise again but lets face it - any Wesley Crusher or Deanna Episode was rubbish and don't even get me started on "Shades Of Gray". Also, how many 'I want to be human' data stories did the series really need? Not enough action, too much talky talky and not enough photon torpedoes
There were some gems of course, BoBW, most Q episdodes for example. Generally I feel that it was developed for a younger audience than the other series. It did get darker (and better) in the last couple of seasons.
4. Voyager - tedious for the first 2 or 3 seasons on the whole but did get better when they were getting a rough ride in the delta quadrant. Janeway was a bit too starfleet for me (if that makes any sense!). Enjoyed the family spirit on the ship though - more realistic for me than TNG. I think this show was an opportunity to do more strangeness/mysticsm in the universe and it didn't really deliver there.
3. TOS - scores here for originality and grunt. No politics here. Mr spock , Bones and Scotty - need i say more. Shatner was (and is!) an arse though - sorry!
Introduced a generation to Sci-Fi - not a bad thing I would say!
2. Wait for it...Enterprise!!! Yeah I (uniquely it seems) liked this show. I thought it did a good job once the first season finished. I enjoyed the arc about the alien consortium wanting to wipe out earth. The sulaban were an intersting adversary. I also preferred it when the Klingons were the bad guys! Also I loved the effects in Enterprise - some real class stuff there (sorry CGI is a hobby and so I appreciate these things more than I should). Liked to have seen it develop further with the Vulcans getting a taste of their owm medicine.
1. DS9. Easily the winner. Decent characters, the fact that it was fixed in in one place was a strength for me as it could devlop the arcs about the Cardassians, the prophets and the Dominion war. I loved Dax and Kira. I thought the mix in the team was believable and in terms of overall picture DS9 had the strongest cast in terms of quality. Oh, it had cool ships, battle sequences and of course the USS Defiant. The grittiest of all Trek. nuff said.
So, if somebody pointed a gun to my head - this is the order I would watch them in.
Fascinating to read everybodies preferences - just shows how different we all are.
Cheers
SteveR