If you have the patience, fishing. Aside from very little startup cost, it's pretty much the only "craft" that you will always make money on. It may start out slow, but at the higher lvls (especially when you get Ebisu), you can make millions a day in NPCable fish (Hakuryu NPC for 10k+), more if you have buyers for things like Lik, Gugru, Abaia, Matsya, etc.
Plus you'll be able to lvl another craft along side of it with the profit you make, should you decide to do so.
Edited, Jul 28th 2012 3:08pm by Reiterpallasch