Something are good because they are good, and somethings are good because they are something that others are not. For me, Overlord is good not because I feel it objectively shines above all other in terms of story and characters (though I feel they are certainly above average), but because it scratches a very specific craving I've developed over the years with excessive anime consumption.
To put it short, I've had my fill of holy savior protagonists, the messiahs, the good boys, the infallible morally just protagonists. I am also sick to death of the edgelords that talk all dark and edgy, yet also never do no actual harm to anyone who isn't literally so evil they eat babies for breakfast. Also I am sick of settings that make antagonists facing these protagonists so reprehensible that killing them is a just act and their assets spoils of war for the side of justice. In short, black and white morality in all aspects.
What I wanted, was either a pragmatist or a hedonist who, upon given powers far greater than those around them, would act as I would assume most people would act if they had the chance: self-servingly. Source: History of the humankind.
Overlord is not perfect, but it scratched that itch of mine that has been growing for years now. Here we finally have a protagonist that does not act as would be morally good, but as how he will reap the greatest rewards. And the world around him does not conveniently make all of his opponents so cartoonishly evil that him killing them and taking all they have defaults to just actions.
