The timing on that one was really poor, not even a season out from the marriage with Worf and then introducing Ezri in the final season. Not to mention the actual death itself wasn't great.
Personally the one that jumps to my mind is Laurie Strode in Halloween: Resurrection(AKA the second worst Halloween movie). Halloween actually got a really good ending with H20 with Laurie killing Micheal at the end...only for that to be retconned, then for Micheal to kill Laurie in the first 15 minutes, then the rest of the movie is a completely unrelated generic teen slasher(with kung fu Coolio). Definitely my most hated big horror movie sequel.
I hated the series finale for MEDIUM. I won't say what happens as it could be a spoiler for someone but it was totally garbage. I'm slowly working through the seasons, but when I get to the end, I'm just not going to watch that episode--I prefer to think it never happened (it was just a dream).
And while I'm at it, I hated that Sally was killed off (off screen) for MCMILLAN AND WIFE that became MCMILLAN, when Susan Saint James didn't want to do another season. With all the cast departures and the show having a completely different tone, they should have just pulled the plug on the series. I prefer to think that final season was in an alternate universe.
Data and some autobots... but they all came back.