As far as I can tell (because I cannot find a full list of rebattle trainers and the prize money they give out), it is Benga, at Black Tower, and Cynthia, with her seasonal battles (only available in spring and summer).
Cynthia's seasonal battles
Benga
There's also N's seasonal battles. He gives out 15000 pokedollars, Cynthia gives out 15600 and Benga gives out 16000. Benga is available daily, N is only available once per season, while Cynthia is available every day in Undella town (spring/ summer).
Another edit: there's also Colress at the Plasma Frigate/ P2 Laboratory, and he will give out 14800.
