A lot I had in mind have been mentioned but also Happily Divorced. I hate when series don't get a proper ending because of cancellation, especially when the last season finale ends with some cliffhanger.
The Young Justice Series Finale pisses me off so much because it wasn't supposed to be a series Finale. It set up so much to happen next season and then BAM.
Cartoonnetwork is garbage for that.
and Honestly Every single AMerican Horror Story Finale Episode has been underwhelming.
The Charmed finale 'Forever Charmed' is by far the worst thing they had ever done! Nothing about the episode made sense. It was though they were writing a 'who can forget what we said five minutes ago' contest. 'Forever Charmed' is filled with time travel plot inconsistencies and character assassinations on multiple accounts. Also the ending came across as someone fangirl's fan fic. AWFUL.