Just for the record, my favorite Star Trek is TNG. Some of the worst but also some of the best actors on all Star Trek, and while the start of the series was rather weak, after that it took up speed.
TOS is kind of a mixed experience. Some episodes are very great, but others are extremely awful. There is not one weak actor and Spock is the best actor on all Star Trek.
TNG had a very extreme cast. Very good actors and very weak ones. And the series required two seasons before it actually really started.
DS9 was a weaker version of Babylon 5. Better sets and effects, but much weaker storyline. The only really good actor was Sisco, the rest was pretty meh, especially the doctor and that model. Also, unlike any other Star Trek, you cant jump in the middle of the show and just enjoy an episode on its own right. I dont really see why this is a big advantage and frankly I dont find the DS9 storylines that interesting.
VOY is the weakest of all Star Trek. Lots of weak acting and weak storylines. But Kes and Seven of Nine have been very fun, as has THE DOCTOR (no not that one), also B'Ellana Torres/Tom Paris and Naomi Wildman.
ENT was a bit mellow in the beginning. But the Cindi - arc in season 3 (mostly) is definitely the best piece of Star Trek, ever. Season 4 was also great. The end was very disappointing and abrupt. I cant help but feel that the casting could have been better. Not that any single actor was bad, just that the mix of actors was a bit onesided. Everyone was pretty super sensible, there was too much of that.