I've been keeping up with One-Punch Man for 4 years now and it is definitely my favorite manga. I'm not really good with essays/reviews so I just simply put my pros and cons of the manga.

Pros
Plot - One-Punch Man plot is very unique as it transforms entirely to a different plot later on in the series. The plot starts out as Saitama seeking out to fight a monster that is as strong as him so he can feel emotion again in his life. This leads him to meet a lot of characters like Genos, Speed-o-Sonic, Mumen Rider, and many more important heroes. The end of this arc leads him to fight an Alien who is deemed the strongest of the universe which Saitama defeats easily. After this arc, the plot transforms entirely. It focuses on an antagonist named Garou who hunts down heroes. His goal is to become a "Monster" but as we find out his true intention is to be viewed as a "Monster" and change the world for the better. He goes through countless battles barely surviving being on the brink of death and his character develops more and more. The plot has transformed so much for the first arc and for the better it is just crazy how the author ONE just change the entire plot of his show and it still is so good.
Characters - My favorite part of One-Punch Man is the characters. Every character is designed and detailed to fit their persona. Just by seeing every character model, you can see the character's personality. The artist Yusuke Murata does such a great job designing every character giving them such depth and detail. The author ONE also develops a wide cast of his characters to make them unique and stand out from the rest. Characters like Saitama, Garou, Genos, Puri-Puri Prisoner, and so many more heroes get so much development throughout the manga. You can tell ONE put a lot of effort into creating these characters.
Art - This is some of the best art in manga. The detail Yusuke Murata does on each panel is just phenomenal. Though this could end up as a con because of the quality of each chapter the quantity of how many chapters we get is a lot more limited. Though I don't mind personally waiting.

Cons Schedule - Downside of the manga is the schedule. In the year 2020, only 26 chapters came out which is an average of a chapter every 2 weeks which seems good but when those chapters came out were inconsistent. Sometimes a chapter could take a week to come out, or 2 weeks, or a month. When waiting for a One-Punch Man chapter it could take a while for the wait. Saitama Gets Left Out - Saitama gets left out a lot in the manga especially in the 2nd Arc. I can see this as a turn down as the manga doesn't even focus on the main character for a lot of the 2nd arc. The focus is more directed on the side characters and the main antagonist Garou. This is personally a pro and a con for me as this leaves more time for character development for side characters but causes the MC to get less screen time. I would give this Manga a 100/100 but Saitama feels more like a side character in the latter half of the 2nd arc.