When you're at lower levels, still relatively early in the game, there is a spot in Junon Harbor that is awesome for exp/gold/ap finding. The location where the soldiers are practicing their routine, in the small corridor. On the left side of the wall, between the two squads, there is a red button. press it to set off an alarm, and there will be loads of lv40+ monsters to battle. Be prepared though; these can be one-hit-killers if you're not prepared!
For the later part in the game, I prefer leveling in the sunken Gelnika. It's probably not the BEST place, but you get a good speed/kill ratio there.
Near the end of Disc 1 prior to getting the Keystone you can take the Tiny Bronco towards the area near Mideel and start stealing infinite X Potions, level up, and try to get enough Limit Breaks exp to give Aerith, Red XIII, Yuffie, and Cloud their Lv. 4 Limit Breaks on Disc 1! (especially Aerith...)
If you're near the end, you actually can leave the Northern Crater (but only once, so make sure you get all the enemy skills and whatnot you want from there to use elsewhere first.) As long as you don't go too far in, you can get ridiculous amounts of EXP, AP and Gil very quickly. To leave, go back to the first area and walk out through the large cave entrance looking door (with Assist on there's a red arrow near it.) If you walk carefully, you can go back up without sliding down the slope. The other great area is in Junon Harbor, the start of the path to the Underwater Reactor- Death Machines can be manipulated, too, which is good for learning Limit Breaks (because they always appear on their own, don't cause status effects and do decent damage, allowing Limit charging quickly.) The area right between the two sides of the elevator Rufus goes into, between the "town" region and the part where the ship docks.