Blue Magic is the most useful magic school on every FF it appears. (yes, that includes FFVI IMO)

Other than Blue Mages? I personally like Chemists (Drink, Mix, Recover and Revive are all useful), Mystic Knights and probably I will love Necromancers when I get my hands on this class.

On a side note, Gladiators - and specially their command Finisher - are overrated and LAME. And Gladiator sprites are fugly. And at least for me, Finisher almost never does Elemental. Mostly I get those "Attempt failed!" or Critical Hits, whch causes good damage, sure, but not enough to... give Finisher to all four characters. @_@