One Punch Man seems to have no limits he can simply destroy anyone with one punch so I am intrigued to see what others think about who could defeat him. Some rules:
1. No time travel as anyone who can time travel would just go back in time and kill him as a child.
2. You cannot put him up against himself as he would still win regardless.
3. Remember it can only be one person against him, even though he could probably take on more at once let's make it fair.
4. Remember that you can have anyone from any universe, DC, Marvel, Anime, anything.