I would suggest you go to the 'White Tree Hollow'. Simply Because the Higher the area, The Stronger the opposing trainers are, meaning more Experience Points your Pokemon will get. As a Bonus, If you defeat Alder's Grandson (Benga) (The Boss Trainer on Area 10, The final Area), You can go to Alder's house to receive a Shiny Gliblet From his Grandson (Once you've beat him). It's also where you go to unlock the privilege of the White Tree Hollow. (By Battling Alder first). It's also a place where you can go to fill up a bit of you Unova and National Pokedex By Seeing them in battle against other trainers. When you successfully defeat the Boss Trainer of each area, You will receive a gift from the clerk at the counter depending on the Area you passed.
From Areas 1-3:
One of these random items:
From Areas 4-5:
Two of these random items (Both the same Item):
Calcium
Carbos
HP Up
Iron
Protein
Zinc
From Areas 6-7:
One of these Random Items:
From Areas 8-9:
Two of a Single Item:
Wing (Clever, Genius, Health, Muscle, Resist, Swift)
Rare Candy
From Area 10:
One of these Items:
Berry (Grepa, Hondew, Kelpsy, Pomeg, Qualot, Tamato)
Rare Candy
You can receive 14 Medals from Mr. Medal For Doing Things In The White Tree Hollow/Black Tower. Here is the Complete list:
- Tree Hollow Junior
- Tree Hollow Master
- Tower Junior
- Tower Master
- 20 Victories
- 50 Victories
- 100 Victories
- 1000 Victories
- Undefeated: Easy
- Undefeated: Hard
- Pinpoint: Easy
- Pinpoint: Hard
- Quick Clear: Easy
- Quick Clear: Hard
If you Prefer to Battle Audinos, I would Suggest going to route one to battle Audions there in the Rustling grass (Since the level Is highest there V.I.A. not including the Black tower/White Tree Hollow.
If you're feeling lazy and cannot be bothered doing work, Simply go to join avenue to level up your Pokemon (Unless your Pokemon Is One level away from Evolving Since they won't evolve there) Or Use a Rare Candy (If you Have one)
Hope I helped! :)
P.S. Sorry for such the Long Answer.