The absolute best way to level is as Frost.
It has the best defenses, it is VERY good for poor gear levels and you will be critting so much it won't even be funny.
It can also solo AoE many mobs at once, thanks to it's massive slow effects.
Plus the pet is a very good bonus, and it is by far the most dangerous in below-90 PvP.
Fire really depends on gear, so i would not advise it unless you have like at least 20% critical strike chance.
Arcane is kinda complicated, it's AoE is not all that good and it's approach to spending a ton of mana for DPSing is kind of a drag during leveling.
Of course this is all just my opinion, but i just recently leveled a mage to 90 so i got some very fresh experience