Area Near Cactuar Island - FF8 Guide
The best location for you to AP grind in Final Fantasy 8 is Cactuar Island which is found in the southeastern portion of the world. You can't reach Cactuar Island with Balamb Garden but you can get pretty close, the best location to park is shown in my screenshot above as well as below in relation to the camp.
Technically the location we're going to isn't even Cactuar Island - it is however close enough to Cactuar Island that when you stand near the edge and open your menu, it'll say Cactuar Island. This is a nifty little well known trick in FF8 which allows you to fight Cactuars while not even being on the island itself.
Cactuar enemies are chosen for this is because they give very little EXP and a lot of AP. If you don't already know the enemies level with you in Final Fantasy 8 meaning that just straight level grinding is actually a bad idea. You'll benefit much more from leveling up your GFs instead of your characters.
Cactuars have an insanely high evasion rating and only Squall has a default 255% chance to hit (guaranteed). Selphie may also have a 255% chance to hit for you if you got her Strange Vision Ultimate Weapon. In addition to a high evasion rate they also have a tendency to flee within a turn or two so defeating them fast is crucial. Luckily they only have 300 HP so one hit will almost always defeat them.
You can max everyone else out in Hit by Junctioning Triple to their Hit attribute if you want to but you really don't need more than Squall to defeat them. Also your characters won't always miss, just like 80 - 90% of the time.
Throughout the game I would recommend visiting Cactuar Island late during Disc 2 and again on Disc 4 to level up your GFs.