1. TOS
2. DS9 (very close call between TOS and DS9)
3. TNG
Although I did watch all seven seasons of VOY, as a whole I though it failed to live up to the promise of its premise, was often poorly written, was too derivative and generic and, with a couple of exceptions, had a less than striking cast.
ENT is the only Trek I have not watched every episode of. I've tried, believe me I've tried. Even samplings of the generally well regarded 4th season don't seem to work for me. My complaints are much the same as for VOY. And that cast? As much as I like Scott Bakula, I think he was horribly miscast and the writing and characterization of Archer did Scott no favors. Although I didn't much care for the Phlox character, Billingsley did a great job with what he was given. As for the rest, I don't think I can even recall many of their names.