Personally I use the Chateau because with Grand Duke Rank you get a writ that boosts the challengers pokemons by 20 levels. Girls that are there wearing Kimonos have Audinos. Lvl 65 Audino with exp share+Exp power 3 brings each fight 15k exp per kill.
Also the Sushi High Roller restaurant in Limouse City is my other spot. Pop a Exp Power 3 before paying to sit down and eat it will last for the entirety of all the battles since they are back to back and no breaks. These pokemon are lvl 65 and give good exp as well.
Also once a day you can fight your Rival in Kiloude City and if you completed the Looker Missions you can fight Essentia in the Looker Bureau.