Manga and Anime-
Attack on Titan wins hands down!!!!!!!!!
The Promised Neverland
Naruto
Bleach
Books-
Stephen King's Dark Tower Series, last 2 books were insufferably bad- (Almost all of SK's endings suck really. His short stories are better.)
Gone Girl
Harry Potter- The Deathly Hallows (for once the movie adaptation was better, though still not great)
Harry Potter- The Half Blood Prince
TV Series-
Lost
The Rain (a series from Denmark I was enjoying until the final season, ugh)
The 100
and of course the bitterly disappointing Game of Thrones
As for movies-
Avenger's End Game was truly awful!