Dungeons:
Dzemael Darkhold (44 - 47) - first room farming is a viable method
The Aurum Vale (47 - 50) - first room farming is a viable method

Leves:
Mor Dhona
Northern Thanalan

FATE's (depending on area activity):
Mor Dhona
Northern Thanalan (Ceruleum Processing Plant)

Low Level Duty Roulette helps too