Naruto: ARGHHHHH! Fillers! Shitty after a while!
One Piece: I hate it just because people are mad crazy about it. Seriously, it looks like some pretty random stuff.
Mahoromatic: The ending ruined both seasons.
Katekyo Hitman Reborn: I seriously liked this anime at the start. It was some kind of a comedy with action, but after a while it became a boring generic shounen with lots of fillers...

Kyou Kara Maoh: This anime had romance/adventure/comedy up to episode 100. After that, it seriously started to give me a generic shounen feeling.
Off-topic but anyway....
Manga I hated the most: Golden Boy (Why? Because, it becomes way too pornographic [nothing against that, but I don't like when a ecchi manga is full of porn] and full of nonsense and rubbish-talk after volume 3. But the anime is damn awesome since it only covers vol. 1)