It may not be the absolute most, but you can get a lot of money by fighting the Bikers on Cycling Road. Giving your Pokemon an Amulet Coin and continually using the VS. Seeker on them all will net you thousands of PokeDollars. You get about 3330 PokeDollars after beating all three, which can add up quickly (and that's not even counting the Amulet Coin bonus).
Beauty Shella on Route 13 gives a bit more money than the Bikers, but her Pokemon are a bit higher levelled.
For the Bikers: Source
For the Beauty: Source and sumwun's comment
Hope I helped!