Northern Crater Leveling Location (Magic Pots & Movers)
The best place for you to farm EXP and AP in the entire game is in two of the rooms of the Northern Crater, the final dungeon. Most guides refer to this area as the Swamp so I am going to call it the same thing, either way you can't miss this area as it stands out amongst the rest of the dungeon.
In this room you're going to encounter two very important enemies, Magic Pots and Movers. The Magic Pots you're going to encounter at a much higher rate than the Movers and that's ok since they give you AP and EXP whereas the Movers just give AP. The only other enemy in this area worth mentioning is the Master Tonberry but not for EXP reasons.
Before defeating a Magic Pot you'll need to feed it an Elixir, if you don't have many Elixirs don't worry as you can use the W-Item Glitch to duplicate as many as you want. If you don't even have a single Elixir to duplicate Steal one from the Master Tonberry enemy I just talked about before.
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.