What locations are the best to farm for experience? I'm looking for those only after starting the World of Ruin and before end-game boss fights.
I like to fight the dinosaurs in the "dinosaur shaped forest" in the World of Ruin when I'm trying to level my characters. This forest is located on an island in the northeastern part of the map, just west of Triangle Island. I believe Brachiosaurs and Tyrannosaurs spawn here.
If you know Vanish and X-Zone, and have the Gem Box (aka Soul of Thamasa), you can double-cast these two spells on them to kill them instantly. The GBA version closes this loophole, but I don't know which version you're playing. Brachiosaurs frequently drop Economizers (aka Celestriads), and you can sometimes steal Ribbons from them, which is an extra boon.
If you're trying to level your whole party, you can leave one high-level magic user in while you gain levels on the others, so that you can power level the rest of the active team. The Brachiosaur casts very high level magic very quickly, so having appropriate speed enhancing relics is also a plus.
If you're not high enough level for this forest yet, you might try the Veldt, which has the advantage of teaching Gau new abilities if you have him in your party and Leap when you encounter something new.
I leveled my characters at the desert just below Jidoor. There are cactuars and Slagworms. Cactuars gives 10 magic AP and 10,000 Gil per round. But you will encounter Slagworms most of the time. These slagworms are very hard to kill if you dont have a good damager and defense. I used Sabin, Locke, and Cyan as my damage taker. So the experience you get is ridiculously high. After you have enough level, make sure that to power up locke and give him the following: Relics: Genji Glove and Master's Scroll Weapon: Ultima Weapon x2 or Light Bringer x2 Armor: (Any armor that have Anti-Magic)
Well it only takes 1 attack command to kill. Make sure ur HP is 9999