Espers. The best magic learning system in a FF game, bar none.
I can think of at least 5 systems that are better.
Sorry, but a system that is likely to just end up with every character knowing everything (no excessive grinding needed either) is not a good system imo.
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MvC3: Haggar/Ammy/Wesker
http://img.imgcake.com/1294368287999pngyz.png