Ranma 1 1/2… started off decent, and then quickly became crap. The ending was terrible; even cancelled series endings were better.
Hikaru No Go was good at first, then the ghost dipped for a stupid reason and it became shit.
Yugioh, the first part was pretty shitty, but after Kaiba's death olympics or w/etf it was called ended, it got better.
Air Gear; started off good, and then it became wtfish. It was like reading a science textbook.
Tenjou Tenge was pretty dumb throughout. I gave up after the main guy's retarded mom showed up and it got all drgaony and shit.
Inuyasha; again, started off good, and then by the 200th chapter i didnt give a fuck. Idk why Rumiko planned to have just ONE villain the entire story through for a 500+ chapter manga.
NANA. I tried reading it. I gave up after realizing everyone was pretty stupid.
Hunter x Hunter, biggest troll manga ever
Zetsubo Sensei, gave up after about 10 chapters.