I like all of the Star Trek universe, and all of the shows have their plus points, but if I had to rank them:
1)Deep Space Nine: Great characters, storylines, recurring cast, everything you could want.
2) The Original Series: Iconic characters, entertaining and action packed SF parables.
3) The Next Generation: Some of the most 'hard SF' storytelling in the whole saga. Very intelligent scripts.
4) Voyager: Always entertaining, action packed, good cast. I like it a lot more than most people here do.
5) Enterprise: Had a slow start, but seasons 3 and 4 were very good.
6) The Animated Series: The actual animation has dated badly, but the stories are very smart for a 1970s cartoon series, and some may have worked on TOS.