I would say that your best bet is the Black Tower/White Tree Hollow since all the areas are build for perfect leveling up, however I will recommend you to battle against the following trainers:
- N (N's Castle): He will battle you once every month
- Cynthia (Undella Town): She will battle you once everyday during Spring
- 2 Game Freak executives will battle you once everyday in Castelia City
- In the Nimbasa Sport Courts, various trainers are ready for battle
- Once every evening you will be able to go on board the Royal Unova in Castelia City, there you will be able to battle various trainers
- Benga and the Black Tower. Once you reach area 10 in the Black Tower you will be able to battle many strong trainers with their Pokemon's level varying from 77-85
- Elite 4+ Champion. Can be battled as many times you desire
Another option is battling wild Audino whiles holding the Luck Egg. You will receive massive Experience points from them. They can be battled in nearly every shaking grass patch