That's a tough one for me, because every Trek series has its own strengths and weaknesses. I know we've done this many times before, but if there's one thing DTF loves, it's arbitrarily ranking things!
Overall, I'd say-
DS9
VOY
TNG
TOS
ENT
DS9 is the most consistently great, and I enjoyed it right from the start, but because of the serialized nature of it, I don't come back to it quite as often for individual episodes. But the payoff to those story arcs was epic.
Voyager is maybe an oddball pick given its inconsistencies, but many of the individual episodes are my favourites, and I just find there's something endearing about it. Once they worked out what worked for them and what didn't (and there were plenty of those), and were able to do more of the science-y CG stuff, I think it delivered many fantastic episodes.
TNG is of course great, but it could be a bit preachy at times, and it got off to a really weak start in the first couple of seasons. Still, it's a classic, and I love that cast and the ship.
TOS is very much a product of its time, but its influence is still felt today, and it established an iconic franchise that has endured for 50 years. When it was good, it was pretty good, and when it was bad, it was hilarious and still highly enjoyable. It was fun and silly.
Enterprise was going through the motions because they had to make another series, and it felt very stale by that point. I thought S3 was really good overall, and is something I will rewatch, but come S4 I felt it just devolved into bad fanwank that was unnecessary and only served to highlight the problems with the entire premise of the show being a prequel.