frost is best for pvp..
fire is best for pve..
arcane is used mainly by pvp frost mages to get improved counter spell. and its used by fire mages for clear casting in raids.
arcane as a main spec sucks for pvp. you can do good dmg in pve but fire is still the best way to go. at lvl 20, 30, 40 ect... i dont really know but once you reach 60 i would say dont waste time with arcane. go frost or fire.