best build... it really all depends on what you want to do with wow or your guild or for money...
solo grinding through the whole game = fire/arcane mage (abilities give them the most solo advantages over all others, free food/water, teleporting, and of course high if not the best survival skills)
money making = beast hunter or a rogue (used for high level instance farming)
pvp/grouping/instances/etc... = millions of options you will need to be more specific on what your goals are