Materia is without a doubt the most powerful system due to the incredible combinations of actions and attacks that can be achieved by linking the right materia and how open to creativity devising such tactics is.
Fire + Quadra Magic
Fire + MP Turbo
Fire + MP Absorb
Fire + HP Absorb
Fire + Added Cut
Fire + Steal As Well
Fire + Magic Counter
W-Magic
Magic Plus
With this combination, Fire (or any offensive spell, really) will be cast upon random targets four times, pausing between blasts while the spellcaster rushes in and steals their possessions. Every blast returns some HP and MP to the caster, and if any enemies survive the magic, they'll be nailed by four regular attacks thanks to Added Cut. Then W-Magic makes it happen all over again. MP Turbo and Magic Plus enhance the strength of the magic, which is probably superfluous, but Magic Counter sets off the combo when the character is struck, meaning that nobody even has to act for it to work.
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Of course everyone knows about Final Attack + Phoenix, which brings the party to as close to immortality as they'll ever need to be.
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Deathblow + Added Cut: even if the deathblow doesn't hit, the added cut certainly will, removing all risk from taking a shot at double damage, which itself becomes triple-damage.
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Cover
HP Plus (or Final Attack + Revive)
Magic Counter + Comet
Counter + Deathblow
Counter Attack (as many as possible)
When the character equipped like this is struck--which he invariably will be, thanks to Cover--the retaliatory chain is set off with a deathblow, Comet, and as many regular attacks as you have Counter Attack materia. HP Plus and Final Attack + Revive keep the character alive, since it's necessary to have him take a beating to activate the chain. Again, this is all set up with counter-type materia, meaning no one needs to take a turn to make this mess.