Lv 60~70 -- Rion Dungeon
Lv 70~80 -- Rion Dungeon, Hollow*
*You'll notice you'll start to get brown mobs as you get closer to 80, but the spawn will still be amazing. As long as you can hold off on your own, the masses of monsters should be able to combat the lower EXP gained per kill. Probably not wise if you're using a Rifle/Pistol, as the EXP:Bullet ratio will suck, but it's still good.