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I liked VI and X because you could get everyone to permanently know all the spells (transferring materia and junctions was annoying).
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The Sphere-Grid system in FFX is one of the best things Square came up to use in a FF game.
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I loved IV's for simplicity.
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I like FFX-2's growth and advancement system, pick a job for all your party mates, then choose what skill you want to first master, master it then choose the next one, slowly expanding out till its all mastered.
alternitively FFX, I love the customization.