From a absolute damage standpoint then Ice3(Blizzaga) is the strongest as a matter of fact here is a effectivity(spell power) list of those spells...

Fire1 = 10
Lit1 = 10
Ice1 = 20
Fire2 = 30
Lit2 = 30
Ice2 = 40
Fire3 = 50
Lit3 = 60
Ice3 = 70

NOTE they all also have an accuracy value of 24 so the chance for them to hit in their higher damage range is equal.