It is not. Now... You will probably say, But, but... It is unique because it does not have the same other animes do not have, Never have we seen a hero struggling to find a villain because he is too powerful!
Well... There is Dr.Slump by Akira Toriyama(Dragon Ball creator) which features Arale who pretty much is a female robot Saitama. And there are plenty of other parody animes too, So not unique in that aspect also.
So now that we established that, What could be the reason for it being so popular and highly rated?
Well lets rate each category without spoiling anything(tho the title of the anime is pretty much a spoiler):
STORY: 5 out of 10 (Mediocre)
It is based on a gag manga, based on a gag web-comic, The whole thing is one big gag, Saitama is too strong and that is funny. The arcs are whatever happens to come in the mind of ONE, They do not seem thought through, They are short and don't make you think too much. Heroes fighting villains, mindless action(not saying that its bad), Has its funny moments, You have a ranking system for both heroes and villains, Kinda like they do in MOBAS. It is not good, But i cant say its bad either, Depends on the person.
ART: 10 out of 10(Outstanding)
To me One Punch Man is visually one of the most appealing animes, All props to Madhouse, They seem like a passionate bunch and it shows.
There are some amazing action scenes in this one.
SOUND: 8 out of 10(Very good)
I liked the voice actors and i like the soundtrack, There are some sick beats to head bob too, It is uplifting.
Characters: 6 out of 10(fair)
Pretty average, Being honest... Not too attached to any of them, Saitama is likable, But all the others seem really generic and bland, I could not give a toss about them, They do not really make me laugh or do not really incite me
to follow their progress, I am sorry, Pretty boring.
Enjoyment: 7 out of 10(good)
I had a good time watching this anime, It is fun and if you are having fun is all that matters for your time spent to be worth it.
OVERALL: 7(GOOD)
This anime is okay by my standards, But it did not meet the hype expectations, I am the type of guy to give everything a chance to impress me and while one punch did have its impressive aspects it is also very dull and bland in others as aforementioned.
