Both mining and herbalism make good money. If you pick it and sell it, you make money. It's that simple.
Enchanting can make a lot of money if you concentrate on disenchanting. This is somewhat dependent on your server's economy, and your desire to spend time at the AH bidding and buying greens and blues to disenchant (I de on both factions and bid on 400-600 items per day).
For a low level, I'd start with mining. Even copper sells. Combine whichever gathering profession you choose with skinning, and skin as you level.
Remember, you can always change professions later on.
To answer your question, there is no clear winner. All three can make a ton of money. As to which one wins is dependent on your effort and your server's economy.
Edited, Feb 19th 2010 7:34am by dadanox
Wahu - Disc Priest
Get an Authenticator!!