I like training at the restaurants of Lumiose City. You get a bunch of experience from battling the trainers here, making your Pokémon level-up quicker. Not only that, but depending on how well you perform in each battle, you also get rewarded with a variety of mushrooms. If you sell these mushrooms to the store, you end getting a lot of money. The tiny ones may not give you much cash, but the rest do. (:
I like training in Battle Chateau, too, which can be found at Route 7. At first, you start off with easy opponents. The more you climb up the ladder, the harder the foes become. Once you reach the top, you eventually start to get challenged by the Elite Four members. Their Pokémon are quite strong. Each time you defeat one, you end up getting a lot of experience. And if you equip your Pokémon with either Luck Incense or Amulet Coin, you get a ton of money as well.
From time to time, I also battle a few wild Pokémon on Victory Road. Each of these Pokémon are near level sixty, so if you defeat them, they provide you with a chunk of experience. If you mange to get yourself into hordes, then great—you could gain even more experience! Battling against these Pokémon boost your effort values, too, which is a plus.