While there are many things I could say about One Punch Man in relation to the themes, characters, production quality, and plot; ultimately, this show's one crowning achievement is its dry, persistent humor. This show answers the question that most shonen brings up but never answers: once you've become the most powerful hero ever, doesn't life get boring?
The art is in a classic style, and the plot rather like what you'd expect of a shonen-style anime. That is to say, there's not much that's remarkable about either. The drawing style is consistent with distinct lines and bold colors, which is pretty typical in an anime of this genre. In terms of technical value, this anime is pretty average.
Yet it still ranks very high on my list because it has achieved a great level of entertainment factor. I really appreciate the characters. They're just the sort of quirky specimens one might expect to find in a show about a man who can defeat any monster with one punch. The villains are suitably obnoxious, but generally have laughable backstories. The heroes are likewise simplified, but in this caricature are profoundly understandable. I would watch again.