E PUNCH!!!!! Saitama knocks out every threat, no matter how big or small. He is the strongest hero, and I personally think Goku, Superman and the Hulk couldn’t touch him (leave your replies below, I am ready…). He doesn’t even try to fight, and he himself knows it, which makes it boring not to find a challenge.
As the series progresses, Saitama discovers that there’s a Hero Association that he can join to get registered and recognized for his triumphs over evil. After doing so, heroes are placed in various classes, starting at Class C, then B, A and finally S. Even though he broke all the physical fitness records during his exam, Saitama ended up in Class C. This lowered his self-esteem, which in turn drove his competitive side to rise through the ranks.
Heroes that are at a higher class tend to look down on him, or think he’s weaker than them. They have no idea that his average guy appearance doesn’t reflect his power, nor does he care much about what they think. Instead, he saves the day every time they can’t, thus making them jealous or making them think he cheated.