If you're looking for sheer damage, Purple Magic gives you the best bang for your buck.
However, just about all of the SP moves (Cutlass Fury, Tackle, etc.) crap on Magic completely in terms of damage output.
Magic is pretty much only for utility and healing, save for a couple early-mid-game patches where it's still useful.
Vyse - Cutlass Fury and Pirate's Wrath will be your best single target nukes for the entire game, relative to cost. Rain of Swords is fantastic for clearing larger groups of enemies once your SP gets high enough to use it on Turn 1.
Aika - Delta Shield is one of the absolute best skills in the game for boss fights. Blocks all normal magics (but not skills) Lambda burst is a cheaper, less damage, Rain of Swords. Epsilon Mirror lets her regen SP, so she's the main 'out of combat healer' once she has that.
Fina - Lunar Blessing's regen is more useful than you'd think. Lunar cleansing is okay for some bossfights. Lunar Light is the best healing spell in the game, bar none. As long as Fina is alive, and you have SP, you can't lose.
Drachma - Tackle is decent. About the same damage/sp ratio as Cutlass Fury. Spirit Guard is awesome, it's Focus++ and Guard at the same time. Hand of Fate is useless unless you really have a hard-on for Drachma. It looks amazing though, and is one of the most hilariously voice-acted supers in the entire game.
Gilder - All his supers are pretty meh. I almost never use Gilder, which is a shame, because he's pretty cool.
Enrique - Royal Blade and Judgement are both worse than Cutlass Fury/Pirate's Wrath, but still worth using if you have SP to burn. Justice Shield is godlike. You take half-damage for a round, for a piddly FOUR SP. In boss fights, Enrique should basically just be spamming this.