A big fixture of the original Dragon Ball was the World Martial Arts Tournament, a contest that Goku participated in thrice before eventually winning prior to Dragon Ball Z. It took a back seat for much of the latter series while the grown-up Goku's attention turned to space, the afterlife, and (to a lesser extent) fatherhood. Eventually, Goku and most of his peers would exceed the average power of a World Tournament Finalist (themselves at the time included) astronomically, to the point that what were once some of the strongest fighters on Earth are treated as a joke by the series.
Enter Mr. Satan, also known as Hercule in English translations, though his real name is Mark. Mr. Satan is first introduced as a contender in the Cell Games, cheered on by the world as the winner of the most recent World Tournament, which Goku and his friends skipped. Extremely arrogant and initially unable to comprehend the Z Fighters' cosmic strength, he quickly becomes a comic relief character, meant to be a pastiche of how a real martial artist would feel inserted in the wacky world of Dragon Ball Z.
How Strong is Mr. Satan?
Later on, Mr. Satan becomes an important supporting character. His daughter Videl marries Gohan, and he manages to befriend the unpredictable and ridiculously powerful Majin Buu. The Buu Saga sees him transform from a hilarious egomaniac into one of the series' best characters, his lack of power compensated by his remarkable ability to befriend those he meets. He still maintains his arrogance on the surface throughout the rest of the series and Dragon Ball Super and GT, but overall becomes much more well-rounded.
By all accounts, Mr. Satan looks incredibly weak, but this is mostly because of the insane situations he finds himself in due to knowing the Z Fighters. By real life standards, he is actually an extremely proficient martial artist, and this is what makes his role in the story so significant.
It would be wise to get out of the way that Mr. Satan's strength is likely just outside the bounds of a real human's, and thus he is comically outmatched by nearly every opponent he tries to fight, including Cell, Kid Trunks, Android 18, and of course Majin Buu. By the standards of the original run of Dragon Ball, however, he is much more powerful, exemplified by the fact that he managed to win the World Martial Arts Tournament.
Mr. Satan can move at extremely high speeds, being able to easily dodge the attacks of the much larger Spopovich, and sending him flying from the ring in a single kick. As a display of strength in the Cell Games, he crushed a large rock with his bare fists and managed to break 14 ceramic tiles with a single karate chop. He easily overpowered the gunman Van Zandt, an ordinary human, managing to kick a gun out the latter's hands before he could shoot him. Mr. Satan also possesses enough strength to pull four buses at once.
By real-life standards, Mr. Satan would be one of the strongest people on the planet, if not the strongest, which is what makes his status as a weakling yet another example of Akira Toriyama's comedy genius. By introducing him, Toriyama was clearly making fun of the extent to which the characters he created had surpassed conventional human limits. Mr. Satan's initial cluelessness about the nature of the Z Fighters' power, and their insistence that much of the fights are tricks and special effects, are exactly what a real person might say if dropped unexpectedly into such a situation. He puts the objective absurdity of Dragon Ball Z into perspective, and that's what makes him so great.