#1: The Next Generation (Do not miss list: All good things..., The Best of both worlds, The Inner Light, Darmok)
#2: Star Trek (Original series) (DNML: The city on the edge of forever, Arena (say cheese))
#3: Enterprise, although the whole Xindi thing departs a bit too much from the Star Trek formula. (DNML: The whole arch from the end of the 3rd season)
#4: Voyager (DNML: Anything from the 5th to 7th season, along with a couple of episodes from season 2, one of them being "Death Wish")
#5: Deep Space Nine, not enough thought inspiring for my taste. Lots of action and battles, not enough issues IMHO.
Daniel