Pokémon Diamond and Pearl/Pokémon Mansion

Though this is a good place to visit the first time around, it will be even better later on, after you beat the Elite Four. This place has the works of a good location: Rich Trainers, items to grab, and a good spot for cathcing wild Pokémon.

Trainers
Note that all cash earnings are with Amulet Coin
 * Gentleman Jeremy


 * Socialite Reina


 * Rich Boy Jason


 * Lady Melissa

Before the Elite Four
You can visit two main places inside the mansion: the owner's room and the Trophy Garden. Head to the right and go in the first room to get to the owner's room. In the it you can find the TM containing Swagger. Talk to the owner and he will brag about his mansion.

You can also visit the Trophy Garden, a place to catch wild Pokémon. You can reach it by going straight after you enter the mansion. Here you will find the following Pokémon: You can also visit the maid's rooms, but there is nothing you can get in those.

After the Elite Four
After you beat the Elite Four, the only thing new is rare Pokémon in the Trophy Garden. Visit the owner once a day to find out what type of rare Pokémon can be found.