It depends on you, do you want a money maker? Or do you want something that will help you win?
For my new troll shaman i chose gnome engineering.
LOVE It. I love making dynamite and grenades. Its quite nice for the extra damage it produces and it also is nice for the stun grenades (give you a few extra seconds to heal)
I also have an alchemist, i gotta say, I NEVER use it. I collect the herbs and i sell them. I rarely use them to make potions.
As far as enchanting, tailoring, leatherworking, and blacksmithing. I imagine you only ever get use out of them when you rarely need something made or when someone asks you to make something for you. Keep that in mind when picking.
If your in it for the money, just pick up 2 gathering professions and later when you decide, get rid of one and choose. Never hurts to go that way to get yourself to 100g (mount) the easy way.