Esper
Summon Effect
Viability
Ramuh
Strikes all enemies with Thunder damage.
Great at the Magitek Research Facility. Middling thereafter. Judgment Bolt is weaker than Thundara.
Kirin
Casts Regen on all allies.
Excellent in harder battles. Party-wide Regen for 18 MP is terrific.
Siren
Attempts to silence all foes.
Situationally useful. Can be handy at the Cultists’ Tower.
Cait Sith
Attempts to confuse all foes.
Not a bad idea when up against tougher foes like Hill Gigas.
Ifrit
Strikes all enemies with Fire damage.
Fairly useless. Hellfire is weaker than Fira.
Shiva
Strikes all enemies with Ice damage.
Also rather useless. Diamond Dust is weaker than Blizzara. Can be helpful at Burning House in World of Ruin, though.
Unicorn
Casts Esuna on all allies.
Quite useful if you find yourself up against a bunch of enemies with status-afflicting moves.
Maduin
Strikes all enemies with Non-elemental damage.
Sadly, Terra’s father is pretty useless. Damage output isn’t great.
Catoblepas
Casts Petrify on all foes.
Generally kind of useless, as most enemies vulnerable to Petrify will probably die swiftly anyway.
Phantom
Casts Invisible on all allies.
Nice for when you’re up against hard-hitters who only attack physically. Which is to say, situational.
Carbuncle
Casts Reflect on all allies.
Terrific throughout the game. Amazing at Cultists’ Tower, since your Reflect will negate your foes’ frequent Reflect.
Bismarck
Strikes all enemies with Water damage.
Situationally useful, since Water spells are harder to come by. Slightly stronger than Ifrit/Shiva/Ramuh, too.
Zona Seeker
Casts Shell on all allies.
Much better than it might sound. Many of FF6’s most challenging enemies are reliant on magical attacks, so reducing the impact is vital at lower levels.
Golem
Shields all allies with a block against physical damage equal to the caster’s HP.
Not quite as great as he is in Final Fantasy 5, but still pretty helpful if you find yourself pummeled by physical blows too much.
Seraph
Cures all allies.
Awful, unfortunately. Seraph’s curative magic is quite weak.
Quetzalli
Strikes all enemies by causing all allies to initiate jump attacks.
Since this will hit up to four times, Quetzalli is impressively suitable even at endgame for powerful parties lacking Relics like Genji Glove and Master Scroll. Caps at 40k damage, which is, of course, outstanding.
Fenrir
Casts Image on all allies.
Image is an interesting spell; it causes party members to evade all physical attacks, but there’s always a one-in-four chance it goes away upon the evading. Quite good in physical-oriented fights like Ultima Weapon.
Valigarmanda
Strikes with, Fire-, Ice-, and Thunder-elemental damage on all foes.
The odds are fairly low that you’ll be up against something strong enough for Espers to come into play, and weak enough to suffer normal damage from all three primary elements. Powerful spell, but situational at best.
Midgardsormr
Strikes all enemies with Earth damage.
Somewhat handy to have around, since it’s one of the few rare sources of Earth damage, and it’ll hit absolutely anything unless it’s flying.
Lakshmi
Cures all allies.
Like a somewhat better Seraph, which is to say, still terrible.
Alexander
Strikes all enemies with Holy damage.
Quite good. Its Divine Judgment attack is slightly more powerful than Holy, making it effectively a one-time-use Holy Plus per fight.
Phoenix
Revives all allies and restores a quarter of their HP.
Situationally useful. If you need it, you need it, and it’s great to have it. But Arise and Reraise will likely do you better on a more regular basis.
Odin
Attempts to instantly kill all foes.
Banish does it better, so Zantetsuken isn’t very good. Remember to keep Odin around for a while before transforming it into Raiden, though, if you want to raise Speed!
Bahamut
Strikes all enemies with Non-elemental damage.
Way, way more powerful than Maduin’s version of the effect, and a go-to summon for guaranteed high damage if a fight’s not quite going your way. Very useful.
Ragnarok
Attempts to transform a foe into an item.
This is simultaneously one of the best and worst summons around. For the most part, you’ll find it useless; however, if you’re interested in farming multiple Genji Gloves, Growth Eggs, Cursed Rings, and Megalixirs, there’s no better way – it’s just reliant enough on luck that you may not feel like bothering. Up to you!
Crusader
Strikes all enemies and allies with outrageously high Non-elemental damage.
Over twice as powerful as Bahamut (though still limited by the 9999 damage cap), Crusader will surely put the hurt on your foes, but unless you’re properly prepared, it’ll also wipe your whole team. Preparation can be bothersome, too, and you’ll likely have easier ways to do that much damage by the time you get it. Woefully useless.
Raiden
Attempts to instantly kill all foes.
This has a higher chance of success than Zantetsuken or even Banish, so it’s awesome for zipping through fights in the endgame dungeons if you don’t mind the steep MP cost.