1. TOS -- I grew up on Star Trek re-runs in the 70s, and the movies of the 80s. Everything else pales in comparison.
2. Enterprise -- This show, in my opinion, did the best to capture the feel of the original series. The last two seasons were certainly the best, but I watched and enjoyed almost every episode.
3. DS9 -- Like others have said, DS9 was more real than the ultra-utopian TNG. The storylines were stronger, and I thought the Dominion War arc worked very well. To me, it was the frontiers of Rome against the barbarians (both foreign and domestic). I liked that premise.
4. TNG -- It was a good show, and I watched it because it was Star Trek (and filled the gaps between TOS movies). I missed a parts of the first six seasons, due to Navy commitments, but I did enjoy watching the series, partly because it was different from TOS.
5. Voyager -- I had actually looked forward to seeing this show, but the horrible inconsistencies in the characters and the reasons for the addition of 7 of 9 made me step away from the show during the fourth season. I did pop in and watch the occasional episode, and I made sure that I caught the series finale.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz