Go for levels 53, 55, 61, 67, 71, 73, 77, 79, 83, 85, 89, 91, 95, or 97.

The Walk of Dancing Shadow in the Feywood is a great place to make Gil by chaining Mirrorknights.

In terms of experience the Necrohol of Nabudis, and the Lsusu mines extension (where you have to chase Gilgamesh) are among the best spots right now.

Later you can unlock the Henne mines extension and Pharos Subterra which provide more experience (although the monsters there are tougher than even the final dungeon).