Rather than Action, *ACTION* fits this manga better.

Story-8
The story are quite generic. Simply put, it's a story about heroes battling against villains. Onepunchman add gimmick such as hero rank system, which shows how the heroes regulated by government. Nothing mindblowing but for an omnipotent hero like Saitama, this actually could build a sense of growth which I don't expect from one-hit-kill main char.

Art-10
Yuusuke Murata providing a generous amount of action scene for this series. No really, when you see action it means real ACTION. 18-20 page for weekly release are just not enough to capturing the awesomeness that put other battle mangas to shame. Arts, details, drawing techniques; IMO it's handsdown the best among manga, or (on some chapters) even comparable to high-end OVA.

Character-9
The diversity of character in OPM is enormous. Major character are consist of heroes though since most monsters/villains are gonna be one-hit-Ko'ed anyway. The main char Saitama portrayed as somehow indifferent hero, neither openly shows hot-blooded sense of justice nor having no justice at all. This generates a pivot point for diverse kind of heroes that appears in the story.

Enjoyment-10
I highly enjoy this series. Rather than push pull battle scene which showing U-turn win-lose-win pattern, it just show how the protagonists win the battle. If 1 hit is all we need to beat the opponent, why do we need to throw new fancy techniques each chapter just to be countered by the opponent on the next chapter? Even 1 hit kill could be interesting if it compensated in another department, on this case OPM leads huge margin by the detailed drawing of battle scenes.

Overall-9
An exemplary of what a battle manga could achieve. So smooth that you could breeze over the chapter in just a few seconds, but without reducing reader's interest to re-read again.