Gamma. Could have been an absolute masterpiece of a manga, but a good third or even half of the story got skimmed over. The result is a travesty of an ending.
I didn't think the ending was that bad. Sure, it was obviously rushed, but an attempt was made to resolve all the subplots and bring the story to a proper conclusion. Compared to the ones I listed, I feel Gamma was ended with relative grace.
You're right that it wasn't too bad really. I meant more that it could have been near perfect from top to bottom, and it ended up with a weak final part, which to me was super disappointing.