Some Marvel fanboys would say it's Hulk, with an approximate amount of 1100lbs-1ton muscle, depending on the source you'll search in. According to Marvel, he can perform any force power when facing a challenge, especially if Bruce Banner can't exert any kind of control over him. Hulk once moved plate tectonics.
Otherwise, on DC's side we got Superman. This alien 220lbs pack of pure Kryptonian muscle is an eco-friendly superhero, since his cells are capable to absorb solar energy. Therefore, this power allows him to have superhuman strength/speed/hearing/vision and heat vision. He's already carried a planet.
So, which one of them has the strongest punch? None. This status goes to a DC Comics' superhero which is a lot smaller compared with these two: Flash. And he's about 187lbs.
Physics explains it all: according to Einstein's theory of relativity, the energy of an object varies depending on its mass and speed. At lower speeds (for example, up to 70% of the speed of light), this isn't too drastic; and we'd be generous to apply this "speed limit" to Hulk and Superman.
Talking about speeds such as the light, which Flash can easily reach, things get much more interesting. The energy he's moving can be described such as the following formula:
(sorry for the crappy edition)
The closest to the speed of light, the smaller the fraction factor and the greater the amount of stored energy. That's why speeding up atoms in the Large Hadron Collider spends so much energy - 120MW in general, and that's enough to keep a city of 120.000 inhabitants.
If Flash can reach the speed of light (or even overcome it), doing whatever is it as he does, he could reach infinite power on his hand. For example: if he accelerates to 90% of the speed of light, he'd have a punch as energetic as A THOUSAND Hiroshima's bombs, assuming his fist's mass is 0,5kg.
And again, that's not even close to Flash's potential. Just replace v/c and see it for yourself.
His punch would cause an explosion in Superman's pretty chin - and this one could reach the amount of 4x the asteroid that would have destroyed dinosaurs. However, we can't tell if Flash would survive his own punch.
Sorry for this whole text (and if it's in the wrong topic - I really didn't know where to put it - and if I've mistaken any word or expression), but I needed to explain very well. It was translated and adapted by myself, so all researching credits goes to Nerdologia channel on YouTube.
If anyone survived this post, please let me know!