The user's arms and upper body are either abnormally strong, physically unique, or have been weaponized in some way (claws, blades, projectile weaponry, protective armor, etc.) to increase combat performance with those particular limbs.
Majin Boo (Dragon Ball Series) punches so hard, he blasted Babidi's head clean off.
Monkey D. Luffy (One Piece) is well known for delivering powerful punches.
Ideo's (One Piece) fists have an incredible destructive power Sugilite that can combine both arms and deliver a massive explosive punch.
Bartolomeo (One Piece) using his Barrier Punch.
A proficient boxer, Charlotte Oven (One Piece) possess incredible power in his fists,...
... increasing it with the Heat-Heat Fruit/Netsu Netsu no Mi's powers as he gives his opponents a brutal beating with his Heat Beating.
Saitama (One-Punch Man)
Gildarts Clive (Fairy Tail) uses his Crushing Evil, Spreading the Truth: Empyrean to punch Bluenote all the way up into the highest of skies.
Natsu Dragneel (Fairy Tail) uses his "Fire Dragon King's Demolition Fist" to utterly destroy a God of War.
While in Assault Mode, Wall Eehto (Fairy Tail) uses his unique robotic arms to deliver struck blows or strikes.
Kazuma (s-CRY-ed) posses an Alter power that allows him to reconstruct matter over his right arm, forming the matter into a powerful metallic alloy, creating his Shell Bullet and increasing his strength...
...he is also capable of increasing his power by using his Shell Bullet Ultimate.
Jonathan Joestar (JoJo's Bizarre Adventure Part I: Phantom Blood)
Joseph Joestar (JoJo's Bizarre Adventure Part II: Battle Tendency)
Jotaro Kujo's Stand, Star Platinum (JoJo's Bizarre Adventure) is a famous user of powerful punches…
…smash and break the diamond-like teeth of Rose/Midler's High Priestess…
…and completely destroyed DIO's Stand, The World during their final duel.
Kenshiro (Fist of the North Star)
Falco (Fist of the North Star)
Ein (Fist of the North Star) using his Kenka Kenpo/Brawling Martial Arts.
Subaru Nakajima (Magical Girl Lyrical Nanoha) uses Belka magic in the form of hand-to-hand combat to amplify the strength of her Revolver Knuckle.
Itsuka Kendo's (My Hero Academia) Quirk, Big Fist, allows her to increase the size of her hands.
Kamui (Gintama) punched Nobu Nobu so hard he sent the man flying out of his tower.
Sakura Haruno (Naruto) is well known for her powerful punches.
Kakuzu (Naruto) using Earth Release:Earth Spear to harden his fist to enhance the physical power tremendously.
Ri Shin (Kingdom)
With strength rivaling that of Ren Pa, Ba Nan Ji, the Demon of Ganmon (Kingdom) shows it off as he attempts to kill Ou Hon.
Rai Do (Kingdom)
Tokita Ohma (Kengan Ashura)
Hibiki Tachibana's (Senki Zesshō Symphogear) Armed Gear, Gungnir, can manifest in the form of a fist...
Clark Kent/Kal-El/Superman (DC Comics) destroying meteors with his bare fists.
Billy Batson/Shazam (DC Comics) demonstrating the strength of Hercules on Superman's face.
Robert Reynolds/The Sentry (Marvel Comics)
Luke Cage (Marvel Comics)
Danny Rand/Iron Fist (Marvel Comics) demonstrates his ability by punching Ragnarok, an evil clone of Thor.
Victor Alvarez/Power Man (Marvel Comics)
Simon Williams/Wonder Man (Marvel Comics)
Peter Parker/Spider-Man (Marvel Comics)
Kamala Khan/Ms. Marvel (Marvel Comics) inflating her fist.
Knuckle the Echidna (Sonic the Hedgehog)
Mister Shifter (Sonic the Comic) attacks Sonic with an enlarged, hardened fist.
Hellboy's BOOM! (Hellboy/BPRD)
As the strongest Viltrumite hybrid of his generation, Mark Grayson/Invincible (Image Comics) possess incredible power in his fists as he gives a Kryptonian like Superboy Prime a bloody nose while trapped in between dimensions...
...pummeled Conquest ...
Thragg (Image Comics) critically injures and nearly kills Oliver Grayson/Kid Omni with a simple punch.
Experiment 601 "Kixx" (Lilo & Stitch)
K.O. (OK K.O.! Let's Be Heroes) combining his and T.K.O.'s power into one enormous punch.
Starfire (Teen Titans) covering her fists in her own energy.
After sticking it in the Cauldron of Life, Owen Burnett's (Gargoyles) left hand was petrified in the shape of a clenched fist, which he uses to his advantage to deliver powerful punches to his opponents.
Megatron: Dark Claw Mode (Transformers: Cybertron)
Optimus Prime: Savage Claw Mode (Transformers: Cybertron)
Renet (Teenage Mutant Ninja Turtles 2012 TV series)
Jenny Wakeman/XJ-9 (My Life as a Teenage Robot) ready with her Fists of Fury.
Frosta (She-Ra and the Princesses of Power) can enhance her punches by incasing her fists in ice.
Glimmer (She-Ra and the Princesses of Power) can infuse sparks into her fists.
True to his name and programming, Ralph (Wreck-It Ralph) is capable of using his powerfully giant hands to wreck anything that gets in his way.
Yang (Yin Yang Yo!) with his Paws of Pain.
Tigress (Kung Fu Panda)
Rex Salazar (Generator Rex) using his Smack Hands.