Mideel (Disc 2) Leveling Location
Mideel is probably the most well known leveling location in Final Fantasy 7. I would be very surprised if there is even a single thing I can say about this place most people don't already know. You'll first be able to access this area on Disc 2 after you obtain the Highwind, however there is an island by Mideel you can reach with the Tiny Bronco that has most of the same enemies. For more information about this spot head over to the Disc 1 Mideel Grinding Location.
Almost all of the enemies in the Mideel area give you a good amount of EXP and AP but a few in particular are going to be your biggest earners. Head Hunters, Spirals, and Crysales will be the guys who earn you the most AP per battle. Spirals may earn you as much as the other guys but they're limited to 2 per battle in my experience.
With the Head Hunter and Crysalis enemies you'll get three and sometimes even four per battle. For this reason I recommend that you grind in the forests only around Mideel. Spirals aren't really worth your time since you only get two per battle - plus there is a flying enemy you can encounter in the grass too that gives even less EXP and AP.
Now would also be a really good time for you to work on Limit Breaks for each of your characters. If you don't know how Limit Breaks work, it's pretty simple... The first Limit Break of each tier is learned by the amount of enemies that character has defeated, the second tier of each Limit Break is learned by how many times the first tier Limit Break was used. For more information check out my Limit Breaks Guide.