For the western folks, I'm gonna stick my neck under the chopping block and say Clone High. Wait, hear me out. They knew they were getting canceled, so they wrote us a final episode that both left things on a murderous cliffhanger, and yet left me somehow okay with that. Think about it, exactly what would happen if everyone got unfrozen? Bad things, that's what. Which is why they ended it in a state of suspension; everyone's just gonna be frozen there forever until someone tries to pick it up again. It's an oddly comforting thought to me, and I have no idea why.
For the eastern folks, Puella Magi Madoka Magica. If you found it too nonsensical, watch the entire series again. If you found it too happy, you are a cruel and evil person and you're getting coal in your stocking this year. If you didn't find it happy enough, I wonder where you were the last 11 episodes.