If every single Pokémon (besides Smeargle) could learn Sketch only once, who would benefit the most? With every Pokémon being able to get one move of choice, the possibilities are endless. For example, Garchomp could have Dragon Dance or Precipice Blades, and Aerodactyl could have Brave Bird or Head Smash.
What broken monstrosities would this change create, /vp?