The user can utilize the Omni-Arts, a specialized system of techniques that revolve around the utilization of omni-power (everything, all powers, etc.), generally depicted as the sum total of all other supernatural arts. This set of skills enable the user to essentially control and manipulate virtually anything, from space to time to reality to the elements to energy to truth and much, much more.
All incarnations of the Avatar (Avatar series) can learn and combine all existing bending arts, on top of learning more esoteric skills like healing and metalbending, and being the only modern human practitioners of energybending.
Aquarion Logos Genesis (Aquarion Logos) is able to infuse anything/everything into its blade when performing its Universe Sword technique.
Sakkakumon (Digimon Frontier) can learn and combine the techniques and attacks of all ten Legendary Warriors.
Susanoomon (Digimon)
Majin Buu (Dragon Ball series) can use Mimicry to learn and copy any/all techniques after witnessing them, which he demonstrates by using Gotenks' Super Ghost Kamikaze Attack...
...and making them capable of the Kamehameha...
...and the Masenko.
The Fierce Deity (The Legend of Zelda) possesses the combined power, memories, and feelings of all people inhabiting the land of Termina, granting him Raw Power rivaling Majora's Mask...
...and enabling him such feats as cleaving a moon in half.
After saving the spirits of the four Champions from the Divine Beasts, Link (The Legend of Zelda: Breath of the Wild) receives a portion of their power, allowing him to use all of their techniques...
Empowered by God, Garou (One-Punch Man) uses Nuclear Fission Gravity Knuckle to punch with the omni-forces of the universe.
Sieghart Caesar (Rave Master) wields all elemental magics, using all 8 elements as well as non-elements including Quintessence/Aether, void, poison, energy, plasma, time and space, and celestial.
As a Mirage Pokémon, Mirage Mewtwo (Pokémon) can learn all Pokémon moves and even use them simultaneously.
Mew (Pokémon) possesses the DNA of every single Pokémon within its body, allowing it to learn any move.
MechaMew2 (Pokémon Live!) can learn any and all Pokémon moves and amplify their power.