I'm forced to disagree with this one. The question was how to make the most OP main character. Not how to sneak an OP character into your story and make them relatable.
This is not even proper advice on how to make a good story despite an OP main character. This is just a stylistic choice and how to put a rug over the fact that your character is OP.
Some of the best anime with OP protagonists in the past 5 years have been Overlord, One Punch Man, and Mob Psycho 100. None of those three did what you say here. In fact, the protagonists of those respective series were very openly super OP and in the case of the later 2, the premise of the entire series is the fact that the MC is OP to the point of being undefeatable.
So, we have a pretty good example here in the form of these 3 sources on how to do OP right. It is to give the MC something different to struggle with. The act of beating the enemy is clearly not a struggle for them, and struggling against a powerful opponent is a cheap source of conflict anyway. So, the MC being WAY too OP to ever struggle against an enemy forces the writer to come up with other more interesting sources for conflict for the MC.
In the case of Overlord, Ainz struggles with his image. He struggles to be seen as a good leader and to please the NPCs that were turned into real people. And, in pleasing them, he is dragged down the path of evil. In the case of One Punch Man, Saitama struggles with the crippling depression he gets from having already overcome every other struggle. This eventually leads to him getting into video games, where the stats of his character are nowhere near his actual physical stats. And, in the case of Mob Psycho 100, Mob struggles with his desire to be accepted and to make friends. He is the most powerful psychic in the world, but he just wants to be normal and would rather not have those powers at all. This leads to him having to stand up and protect those friends once he makes them.
So, work-place image, depression, and highschool friends. All three of these popular anime with OP protagonists build incredibly relatable OP protagonists by having them really struggle with something very relatable to the average person, and their OP powers do not help them at all with these struggles. In fact, if anything, they make them worse. (In the case of Saitama, his OP abilities are actually the source of his depression.) Not many people can relate to the struggle of having to overpower the villain in order to save the kingdom. I would say zero people can relate to something like that. However, people really enjoy seeing an OP hero who can kick everyone's rear without breaking a sweat turn around and struggle with something they can totally relate to. Seeing those relatable struggles allows them to identify with the character, and that in turn makes the reader who is already identifying with the MC get an even bigger thrill out of it when they start bashing some heads like the reader wishes they could do themselves.