If you're into mindless grinding - spiritbonding is probably your best bet. Grab some vendor gear from Mor Dhona, go to Urth's Fount and join a spiritbonding party there, pop your SB potion, and SB for 3 hours. Crafting/gathering materia tend to give you stable returns, but with battle materia, you can either hit the jackpot, or not come close - pick your poison.