In WoW what talent tree is best for mages

It depends on what you want to do.

For PvP (player versus player) go down the frost tree. Frost is all about slowing down your attacker while you run around throwing frost bolts and fireballs.

For raiding the fire or arcane tree is best. Arcane is better for mana management as there are ways to get mana back and reduce damage to yourself. Fire has some big numbers but is more of a mana hog.

Of course this might change when Cataclysm gets released. Blizzard is changing the way the trees are made up and how talents work (lowering max talent points to 41 and requiring 31 points in one tree before selecting talents in a different tree).