If you're reffering to the best place to gain sphere levels, it would be the omega ruins. The enemies give great AP, especially if you have a weapon with Double or Triple AP, and the enemies drop equipment that you can sell for a good chunk of change. Also, you can fight mimics which are good for 50,000 gil a pop, which is really good later when you have to bribe enemies for the rare goodies. Also, if you equip someone with Gillionaire, you'll get twice the gold, and then mimics are tastey treats.