The three star restaurant is where I go.
If you bring an amulet coin, you'll make back most of your money just from the battles. Then you sell the balm mushrooms for money.
Make sure you do triple battle though. That way you can have three pokemon getting 100% of the experience.
It's much faster than doing the elite four over and over, since you don't have to watch the credits. On top of this, the pokemon are elite four level.
