I had the most fun casting magic in FFVI, but I don't think that was the 'greatest' magic system in any of the games. I really don't know what the greatest might be, but tactics can certainly give them all a run for their money.
FF I
FF II
FF III
FF IV
FF V
FFVI
FFVII
FFVIII
FF IX
FF X
FF XI
FF XII
FF Tactics
FF Tactics Advance
FF Crystal Chronicles
Other Spin-offs... (Dirge of Cerebus, Mystic Quest, etc)
I had the most fun casting magic in FFVI, but I don't think that was the 'greatest' magic system in any of the games. I really don't know what the greatest might be, but tactics can certainly give them all a run for their money.