frost for multibox leveling and PVP - for multiboxing probably dump it all into frost, frost PVP has just 1-2 viable builds that combine arcane (TOTW/focus magic) plus frost
FFB or arcane (57/3/11 i believe) for starting solo raiding/heroics
fire (18+50/3 anywhere) for high end raiding (2k+ sp, 360 hit, 25%+ unbuffed crit minimum)