I don't know that there is a best. Gathering professions are generally more profitable, but I don't count them as a trade skill. Enchanting lets you both gather and develop a trade skill, so I guess I can default to that. They are all good if you know how your realm is developing so you can anticipate what to market. Though Engineering is easily the least profitable trade skill from my experience.
Edited, Jun 13th 2007 5:53am by Vivainne