One punch man has HERO ASSOCIATION, while hunter x hunter has HUNTER ASSOCIATION
Both of them tells about character's unique power.
Both animes are from the same studio. Meaning they look awesome, just AWESOME.
The fights are easy to follow, look amazing and have a lot of detail.
Some characters looks really remind you of another in a positive way.
Both animes have a goal to aspire. Becoming a great Hunter or a great Hero.
They involve different species and a complex varied world.
Many of the characters from One Punch Man and the setting reminded me of Hunter x Hunter. If One Punch Man weren't a parody, had a lot more episodes, and if Saitama weren't OP, you would almost have a Hunter x Hunter twin-series.
Both anime are done by Madhouse and have the same overall feel to the series - unique characters, epic battles and power-ups, etc. The "Hero" system in OPM and the "Hunter" system in HxH are very similar as well. Both are incredibly good shounen series, and while OPM is more humorous, there's a high chance you'll enjoy one if you enjoyed the other.
Both have some of the same themes like an association of professionals (heroes/treasure hunters) with class ranks and an elite set of very strong people and the main character eventually decides to work his way up the ranks and become more well known and stronger in the process
Both shows have an organization that the main protagonist joins to fulfill some sort of goal. The Hero Association and the Hunter Association in the two shows remind me of the other. Both have a ranking system which dictates what level of jobs you are likely to be allowed to complete. They involve a rigorous test which has a small passing rate. Also both organizations send out their heroes/hunters to battle and take care of weird creatures that could pose a threat to humanity.
Another comparison is that that main characters have a similar altruistic attitude towards helping people. All in all, these are
Both have equally crazy fights and whacky characters. OPM opening also parodies HxH's main character, Gon (the punch where he holds his left hand over his right before punching).
Both have probably the most brilliant soundtracks ever also. You really need to experience these two. And of course, both are by Madhouse Studio.
Both are fun and interesting takes on less conventional superheroes, though in Hunter x Hunter they're called Hunters rather than superheroes. Both have a diverse and spectacular cast of minor characters. Hunter x Hunter is more emotional, but both have great comedy and are a joy to watch. Both have a lovable main character and a lovable side-kick.
Most of the characters share a few traits: they seem like normal human beings, but they bear very special talents that might not be recognized in the real world. If you love laughs, drama and the struggles of being “different", then you will love this as well.
Tournaments, different powerstyles and various heroes