For me, it was Hate Mate. Because until the very last panel, I truly didn't expect these two to end together at all, not for a second, it never even crossed my mind. I was 100% sure it was gonna be the other one. It's even more shocking that it seems like I'm the only one siding with the other character. And I still think choosing him would have been the better end. But whatever, I don't really care... everyone is happy..... except meee TT
Ditto on Hate Mate. It stuck so bad I can't even reread the manhwa again
Also
https://www.mangago.me/read-manga/truth_or_dare/
Not exactly the ending. But the plot twist mid end had me rolling
What is the end that shocked you the most?