Sakamoto

Sakamoto, from "Haven't you heard? I'm Sakamoto" is possibly the single best use of a mary sue character seen to date.

Sakamoto exists not as an insert character to make a viewer feel perfect within the shows universe, but as a comparison character to show just how awful everyone watching and in the universe is. He is perfect, and thus shows everyone their flaws in perfect detail when they compare themselves to him, which is why everyone dislikes him at first when they cannot handle the aspects of their life and personality they were either ignoring or pretending were fine. He seems to (knowingly) improve people's lives just by being in the general vicinity, and afterwards most of the characters that meet Sakamoto tend to know it was for the best, and usually tend to like and respect him, instead of being filled with disdain at just the sight of him.