I have always thought Saitama (One Punch Man) was unbeatable

But in fact, considering the abilities that Accelerator (A Certain Scientific Accelerator), the most known one being manipulation of vectors, he could easily kill Saitama just by reversing his blood flow or reflect his punches with an unbreakable barrier without a form so he won't be hurt, not even a scratch

In his Awakened form, he could simply kill Saitama just by looking at him