Deep Space Nine is by far my favorite because it breaks the mold of traditional Star Trek. It wasnt just the usual find new planet, meet cool aliens, fight and leave 3-step process employed in much of the other Trek series.
The characters are amazingly well developed and episode plots focus equally well on action and intrigue as well as the more humane personal stories. For once, not everyone is the perfect smiling Starfleet officer. Youve got characters like Garak, the ex-secret operative who now works as a tailor (or does he
.?), and Quark, whos supposedly a capitalist, money hungry sneak who will do whatever it takes to acquire more latinum, but deep down might actually be more honorable than a Klingon.
The entire Bajoran / Cardassian / Dominion / Federation storyline is such an epic war saga, and I remember being on the verge of tears at the end of the final episode. It is a bit hard to get into the series because of the continuing story arc, but well worth it.
As for the others, TOS is wonderful in a this is what cheesy sci-fi should be kind of way. TNG is my second favorite series; the later seasons were absolute quality. I enjoyed Voyager at the time it aired, cheers to them for picking a female captain, but it didnt have replay value. Enterprise, I stopped following after about 3 episodes
Yes, Im a geek