It's seriously the best. You are doing yourself an extreme disservice if you don't sit down and read that Onepunch greatness. The plot's plastered literally everywhere so let's just go ahead with the usual breakdown thing that seems to be the way to go on mal. Welcome to the glorious anti-stereotypical battle manga, ONEPUNCH M
Story-7
It's not gonna blow you out of the water with an extreme level of intricately detailed depth and massive psychological implications, but this is literally one of a TINY number of instances where I couldn't care less if I actually put considerable conscious effort into doing so. Story for the most part flows well and has enough bubbling (or violently exploding) away at any one time to keep me interested.
Art-10
Yeah. 10. It's just that downright astoundingly nice to look at. Character designs are interesting and extremely detailed, backgrounds are amazing and there are literally bits you can string into a gif image and have yerself a small piece of absurdly fluid (coming from a manga at least) animation. Our beloved MC here is the only one who's drawn horribly-but that's part of the joke. He generally looks like he's drawn in Microsoft paint, but when he shifts to a serious art syle you know shit is about to go DOWN. Best art I've seen in a very long time.
Character-10
This is one of those things that's probably hit or miss depending on your tastes, but I personally LOVE these characters. Saitama is our MC and super-OP badass mofo who people don't take seriously cause he's bald and walks around in tights and a cape-you'll probably grow to love him as he makes mountains out of molehills (ie:missing a convenient sale at the shops while killing bad guys) and generally not caring in the slightest about things that seemingly actually matter. There's Genos, the also extremely badass and super-attractive fangirl bait who possesses a plethora of crazy robotic arm attachments for taking care of the toughest jobs, and a host of other heroes ranging from licenceless rider, a dude on a bike who rides around fighting crime, to guys like Bang and Metal Knight, Also super-OP and awesome. Vibrant cast who grew on me faster than most.
Enjoyment-Gotta be 10 man, Onepunch is the biggest breath of fresh air I've had in a while as it stubbornly wages it's one-man war on shonen battle manga. If you're sick of shitty tropes and garbage plot devices, prepare to see them mocked relentlessy in a setting where the action and characters still manage to dwarf most other battle manga.

I'm gonna go with like 9.5/10
This be mah jam.