It's not better than what yall listed but Gantz belongs in the discussion for sure and people might not agree with me but I thought D. Gray Man fell off REAL HARD after a dope first 100 chapters or so and just got worse and worse.
I know that part of it was because Hoshino got sick all the time (though this doesn't keep
Hunter x Hunter from being good whenever Togashi decides to have it published...), but by the time they got to North America, the plot was literally incomprehensible. I'm still kind of confused as to how the manga went from China or wherever the Order was to North America and that whole Alma Karma mess. It just seemed like they went from one place to the other in the space of like 2 panels or something.
I feel the same about Code Geass. Season 1 >>>>>>>>> Season 2 (but the series finale was

)
I'll cut the creators a little bit of slack with Code Geass, seeing as they had one thing planned and then got their timeslot changed on them (they went from a late night timeslot to an early evening timeslot that came right before the evening news, which necessitated content changes because the audience was broader and hadn't been really exposed to the first season of the series. Also, they couldn't do some of the stuff that they were allowed to do in the late evening content wise as well), which made them have to basically redo the season from scratch. Goro Taniguchi was widely reported as HATING the change, which was forced on him. So there were extenuating circumstances.
Gundam 00's second season, or at least the last 15-20 episodes or so? Yeah, they had a lesser version of Code Geass' problem, but the drop off really shouldn't have been as pronounced.
Death Note's problem came from the fact that, in having him outstrategize L in the first half, they made him too clever to plot for. They couldn't realistically have anyone beat him without having him make galling mistakes that he'd have no chance of making during the first half of the series. So it ended up flat mainly because it was unbelievable for the character of Light to be as stupid as he was.
Given the problems of other Manga, however, this wasn't that bad.