Considering your levels, i'd say Giruvegan would be a good option. Try the Pharos-Subterra for a challenge.

After you get to around the upper 60s, venture into the Henne Mines east section. It is quite possibly the most rewarding area in the game. My main party is approaching the 80s, and they still level up rather quickly! Chaining Abysteels is also a great way to recieve loot. And while you're there, try to Poach a Gizmaluke. You may recieve a Holy Lance, which'll come in handy when fighting Zodiark. It'll require several tries though.