valley of the cactuars. PERIOD! with triple AP weapons you'll get 36,000 AP from each one and with a possibility of 3 cactuars in a battle that makes 108,000AP in one battle. plus cactuars drop in one attack anyway.

plus, this can be enhanced even further with overdrive -> AP and triple overdrive abilities.

you may get the odd battle against a sand worm but they have some decent AP too.

P.S just thought of a quick sidenote. if you put double overdrive and triple overdrive on a weapon, do the effects cumulate or is double overdrive cancelled out. does the same apply to double/triple AP?