Dukat's lamentably stupid S7 arc.
I'll think of Dukat's development in season 7 as a sort of time capsule for you to open up some day and discover and appreciate, years from now. The end in the cave wasn't very good, and the start of the season really wasn't either, but despite DS9 treading too close to "magic" territory for a SF show, with the P'a Wraiths (?), the rest with Dukat was amazing.
ST-- Turnabout Intruder... a bad end. Sexist and clumsy. However, since the regular season ended with a great sendoff, "All Our Yesterdays", and since they went through their summer of reruns before showing just one new one (TI) then leaving the air... I've always felt as if TI was a sort of 4th season premiere, for a (terrible) season that never happened.
Therefore, I think of that beautiful final moment of All Our Yesterdays as the end of ST, that tragic, fatalistic winking-out of the planet.... and those responsible for replacing it with that big showy crude CGI effect ought to be in jail.
And I don't like Undiscovered Country. If I chose a Trek film as the original Trek's finale, it would be Generations, where we actually do get to say a goodbye, and which is a much better film, to me. I forget if Spock was last seen in UC or Reunification. Scotty-- Relics, right? That was better than UC too. Chekov? New Voyages?
Nemesis... need to see it again, but didn't seem like Next Gen to me. All Good Things... a lot better than the average later NG episode. Very good, except for the obvious over-reliance of Berman on solving all their problems with beams of colored light, especially from the reflector dish, or whatever they called it.
In Nemesis, that disease doesn't seem to be affecting Picard much yet... unless it is in there and I forgot.
DS9... great. They get into Dominion headquarters in an easy "TV" way, but it's still the best finale.
Voyager... Endgame-- pretty good. Why complain, nice character moments, two different homecomings (I love Voyager in sky over San Francisco), too late to complain about excess time travel at this point, at least this was better than most instances in Voyager....
Enterprise--- who says you can't give zero stars?
All Our Yesterdays, What You Leave Behind plus the story before it (as a 2-parter), then AGT, then Endgame.