if ur at a lower lvl (like 20ish i think)
outside of rocket town around the mountains (where u came from when heading to rocket town) there is some grassy areas...
in there you can run into some guys that give you like 30 ap a piece (they show up in hoards of 3-5) so pretty nice AP and they are easy to kill.

good place for when ur a low lvl

lol

See, leveling in the submarine doesn't only get you to level 99; it gets your stats to 255, making it irrelevant what your level is.

For the most AP gain in one fight, go to the northern cave. I can't remember the particular area, but if you check some FAQs/Walkthrough's, I'm sure they will mention this. There's an area that you can fight Movers, they always come in three's. Sure, they are a little rare, but it was always worth it for me. Using a weapon with a triple growth rate, from one fight, you gain 7200ap. Other than that, go with HeadHunters near Mideel or the Sunken Gelnika

Northern Crater. 2 monsters down there. One gives you 150AP, the other 200AP. 30k gil and 20k gil

i used mideel to lvl mine up

sunken shinra plane, morph ftw, anywhere will level you, but only morphing there will make your chars godly, i thought everyone knew that...