A good ending in a manga often ties up loose ends, shows character growth, and leaves a lasting impression. It might make you think about the story long after you've finished reading. Sometimes, it's a surprise ending that works well and makes the entire story more memorable.
It depends on personal preferences. A good ending could be one that resolves all major plotlines, gives closure to the characters, and leaves you satisfied.
For many, a good manga ending is one that feels logical and consistent with the story's development. It should also have some emotional impact, whether it's a happy or bittersweet one. Also, if it leaves room for interpretation but still feels complete, that can be considered good.