DS9: none (don`t like it)
It's got some great character development and, unique in Trek, a growing continuity/story arc.
The "growing continuity/story arc" is one of the reasons I have not cared for DS9. I have always preferred episodic television.
Lately however, I find myself enjoying some story arcs. Perhaps, I should give it another chance.
I think it's good. The first two seasons are standard Trek fare, and better if you ignore previous series, but the latter two are very good in my opinion. Some continuity in the third season, and an interesting take on story arcs in the fourth.
ENT: none (haven`t watched it much)
Same thing in the case of ENT. When Trek was on television, I had a very busy schedule. Missing an episode or two would be common. Unfortunately, when I came back I was completely lost and gave up.
TOS: Spock (also McCoy and Scotty)
TNG: Wesley Crusher and Dr. Pulaski (Yeah, I'm evil, but I like what I like.)
DS9: Kai Winn, she was evil but I liked the way she was written... at least the episodes I saw. Never got into the series much, except for "Little Green Men" and "Trials and Tribble-a-tions"
VOY: Tuvok, Harry Kim & Tom Paris
ENT: Archer & Porthos, the Doctor
nuTREK: Pike (hate what happened in STID), Kirk (like seeing early Kirk grow into what we know), McCoy & Scotty