Tough to answer. I always like to rank TOS separately, the gap between it and the rest, plus the fact that it spawned all the movies, make it special.
As for the modern ones, I'd probably rank them: TNG, DS9, VOY/Enterprise.
TNG was what made me a Trek fan... sure I'd watched the re-runs of TOS, but this was all new! I still remember the first time I knew I had to be home in time to see Trek. It was after 'The Best Of Both Worlds' and I had to see what came next.
DS9 had variety, and it's own style which I loved. The mix of cultures, Bajorans, the Prophets, the Wormhole... Plus I don't think the Experience could have happened without Quark's Bar and the Promenade!
Voyager and Enterprise had good and bad, but Voyager was let down by definite-end syndrome, i.e. the ship gets home - end of show. So anytime there's a chance they can, you know they won't. That gets old fast. Plus it had to work double hard to do stories that didn't feel like TNG.
Enterprise was let down by the folks in charge. The direction they forced the show just didn't work for he premise of pre-TOS. Personally I always felt there were too many new species, not enough use of TOS species, and ones that were only met in TNG or later had no business being seen that early.. Oh and the temporal cold war was a stupid idea. (but not as stupid as Nero and Red Matter!)
