There is no character in Hunter x Hunter proven necessarily to be as strong as Meruem. He never legitimately lost a fight, and honestly, he never came close to losing one either. Meruem is the Chimaera Ant King.

Hundreds of humans were eaten, their life forces all combined made stronger ants, who hunted insanely strong humans, where thousands of those meals were harvested, to make insanely strong ants, who then were able to kill hundreds of hunters, the strongest of the strong, which when all combined made Meruem. This is the pinnacle of evolution. His tail could whip people's heads off that otherwise were massive threats to general fighters, and it could happen in miliseconds. Calling Meruem a genius is unfair, because his intelligence was well beyond that. Within hours he defeated all of Earth's best players in Chess and other games, except one human, a blind woman. He, even up to his death, never could defeat her.

Meruem evolved overtime. He was an absolutely heartless killer capable of nothing but despair. He valued only getting stronger and not seeing a point to anything that was capable of surviving agains tall odds. As time goes on, he becomes more and more human, not just defending a weak blind woman, but sparing humanity. In his time of dying, he's as human as it gets, valuing love, friendship, and hey here's a thought - non aneurysm inducing hobbies over simply being stronger and smarter. His emotional development may have changed everyone's perception of him, as he was previously only known for his tremendous power; however, it can be said he was in his best and most sacred form when he changed focus.

Meruem is a specialist. He can use literally every type of Nen, and better than anyone else. He can transmorph his body in anyway he chooses. He has over genius intellect. He's stupidly durable. He's just... OP.

To see his most amazing fight, watch Meruem vs. Netero. The fight lasts about 50 minutes. It's... incredible.

And so are you Diamond. Great job today, friend.