WHO GOES TO THE FINAL BATTLE?
So, the game makes you pick an order from first to last (3 being the lowest and 12 being the highest, depending on the size of the party you brought to the tower) a priority order on who fights the final battle. 1 through 4 start the battle then if someone is unconscious when a stage of the final battle ends, they get replaced with whoever is next on the priority list. Say you had Edgar/Terra/Celes/Locke as your first group of four and Celes is unconscious when stage one ends. Say your 5th priority in the list was Relm. Your party for stage two would then be Edgar/Terra/Locke/Relm. This repeats for the transition to stage 3, and then once more for the final stage which is Kefka himself. Naturally you will want your four best to start. Unless you want a challenge, then by all means throw out your four weakest first. So as the heading of this section says, who goes? Well, that depends. Are you playing through normally or are you playing a challenge? Did you teach everyone every spell? Let's look at each character here and I will show my list, from a file where I only taught magic to those who canonically should know how (Terra/Celes/Relm/Strago).
#1 - EDGAR
No final battle strategy for any RPG is complete without a primary fighter. Edgar starts out the battle for me as my top pick for the fighters. He is equipped with Dragoon Boots, Dragon Horn, the Aura Lance, and excellent defensive gear. Your best fighter archetype when you look at equipment proficiency and offense/defense/Health stats together. He likely won't even need any spells as he'll be too busy Jumping, but if you must stop to have him help with healing or reviving, it couldn't hurt if you taught him those spells. I had him Jump the whole fight.
#2 - TERRA
All spells learned, great equipment, and the Morph command. Terra definitely needs to start the battle. Morph when combined with the Quick Spell and the Economizer makes Terra very dangerous for the final bosses.
#3 - CELES
Same as Terra but without the Morph command. Celes should be your primary healer and also needs to start the battle. I had her learn the full spellbook as well.
#4 - LOCKE
Locke is my "true" #1, I placed him at the 4th slot so that Edgar/Terra/Celes will get attacked more frequently due to their better defensive gear. He's here for the Offering + Genji Glove setup. He has an Atma Weapon equipped and will be stealing from both of the bosses of the third tier. If Locke makes it through the full fight up to Kefka with two Atma Weapons, Kefka has reasons to be afraid. You'll want Locke starting or as one of your first backups to ensure he has a chance to Steal the 2nd Atma Weapon and a Ragnarok Sword. He's better off fighting instead of casting in the first two tiers and against Kefka. In tier 3 he's crucial.
#5 - RELM
I had her learn the full spellbook too and she has that great magic stat. So, she was my backup in case I lost either of the two better casters. Make sure you taught her magic though. Had she been relying fully on Control or Sketch, she'd be in the bottom 3 in this listing.
#6 - STRAGO
The Lores will come in handy. Mighty Guard and Grand Train are all Strago needs in the final battle if he gets to participate. Great backup caster even if he is my least favorite character.
#7 - SHADOW
Got enough Ninja Stars to spare? Good, because that's what Shadow brings to the table if you brought him. Good all around backup for fighters or for casting if you taught him magic.
#8 - SABIN
Most would probably rank him higher, but I'd rather use things other than Blitz. Edgar's Jumping and Locke with two Atma Weapons are better than Bum Rush. Not that Sabin's a bad choice to bring, he's not. Make him a backup fighter, NOT a backup caster.
#9 - MOG
Edgar's the better Dragoon and Dance sucks for the final battle. Odds are you're not going to get this far down the list anyway though. Good backup for either a fighter or a caster if you taught him spells, it's just that you have better options on both sides.
#10 - SETZER
Fixed Dice and Dice are unpredictable, but good. I'd rather rely on the other fighters. Whether you taught him some spells or not, he is in a similar situation to Mog in his usefulness in the final battle. There's better options across the board for both fighting and casting. GP Rain is outclassed for area of effect skills by your casters using Ultima, even if you had the Gil/GP to spare.
#11 - CYAN
Another potential pick for a backup fighter. SwdTech is a pain to charge up to get that Quadra Slice ability at the 7th point of the bar. There's better fighters than him and he's a HORRIBLE caster so that's why he got ranked low on the priority listing.
#12 - GAU
The last thing I want to rely on in the final battle is Rage. Gau's also outclassed in casting by the girls as well as by half the guys, so why put him anywhere but last? By this point in the game Stray Cat's Rage is outclassed by your fighters. Sorry Gau, you're sitting this one out.
WHAT ABOUT UMARO AND GOGO?
Glad you asked. Had you brought them, they would be as follows.... Umaro above Setzer.... Gogo above Strago (potentially). Umaro's constant attacking makes him good for a backup fighter in this fight, the downside of course being you cannot make him use items or do stuff other than being berserked. Gogo depends on how you set him/her up. If he/she has Blitz, Magic, and Lore, then Gogo is potentially your best backup character. Gogo can also be a second chance at Stealing the 2nd Atma Weapon and the Ragnarok Sword from the bosses in tier 3 if you lost Locke earlier on in the fight. You may even want to have Gogo start the fight as one of your first four! That is up to you.
HARD MODE NOTE:
You only have Edgar, Celes, and Setzer. Ranking them serves no purpose. All three will participate and you will NEED all three.