Where is the best spot to level my characters in Final Fantasy 12?

Ozmone Plains. Or you can just circuit. Like, travel back and forth from Rabanastre to the Eruyt Village and fight the monsters you meet along the way. That's what I do. Also, cycle your characters. Like, use three characters after they leveled up three times (as in they're all the same level) and then switch to the other three characters.