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.
Along with XP grinding, there is the question of MP grinding. The answer to that is the desert area just south of Maranda. Hoovers are high on HP, but I've found using Offering + a weapon with a instant kill chance CAN end the fight quickly, before he starts using "Crush" to instant kill your toons. 5MP per hoover isn't bad and the XP is pretty good as well.
Also located there are the Cactuars, worth 10MP / kill, but no XP. For an easy time with them, equip the Atma Weapon and give them Sniper Sight relic. The 100% hit rate ensures the hit, and the Atma Weapon does damage based on your character's HP, ignoring all defenses, just like Edgar and his drill, IF he actually hits with the drill. Since Sniper Sight only applies to "fight" command, this combo of weapon and relic guarantees a one shot kill on the little buggers, putting 10 Magic Points in your pocket, along with 10k gold no matter who you put them on. So if you need to learn spells quickly from espers, this method will do the trick. After ten cactuar kills, just avoid the center tile on the lover left portion of the desert, so you don't run into the Gigantuar...unless you're ready for him!
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
Right next to DOMA in the WoR you fight solo, with invisible status... The enemies can't touch you. If you are using 2x experience egg then you will be gaining almost 20k experience or something like that PER battle. You can easily get to level 99 this way within 1-2 hours... In this game one character in the high 90's is pretty much undefeatable. The first time I beat this game I did this with 3 characters and ran Kefka's tower with 3 parties of 1 level 99 character lol.
I've always filled up in the desert south of Miranda for the obvious 5MP (Hoover) or 10MP (Cactuar).
As others have mentioned, the Cactuar gives no XP, but the Hoover gives nearly 2,000 per battle.
What the other answers have missed, however, is how painlessly this desert can be grinded.
I've played this game several times as a kid and am going through it again, and, let me tell you, this trick is as easy as pie as ever.
Firstly, make sure everyone has vanish (summoning phantom makes this part a sinch) before going to the desert. You will only need to do this once.
Then, just make sure you have a person who can use tools (Edgar or Gogo, even better to go half and half with them to split up the experience).
If you encounter Cactuar, pass the turn to the tool user and use the "Drill" for an instant kill. I've never had him 10,000 needles me before he dies, as long as I was paying attention.
If you encounter Hoover, again, go straight to your tool user and hit him with an "Air Anchor." He WILL NOT counter with Sandstorm. Then just wait for him to attack or use Crush (which will miss because you're invisible) and he instantly dies, every time.
You can effectively grind this desert as fast as possible without taking so much as a single HP of damage.
I've only ever played this game on the SNES or Playstation platforms, so I can't speak for the other versions, but on these, this method is flawless.