A tie between Ash's chimchar and litten for me.
Froakie and Pikachu
Ash's Primeape.
But how would Froakie's story be told otherwise?
Sure, Ash could have found one of the trainers who've returned Froakie but I feel like that would make the narrative go to places the writting wasn't interested on going.
Olympia explaining actually worked fine IMHO