There’s a lot of information here but to keep it short.
Because the items that sell for a lot of Gil are hard to make they require cross class skills. Thus, mostly people with all crafters fully leveled and best gear with melds make te big Gil. Everyone else make moderate to good amounts with varying degrees of success depending on how many crafters they have at max level and geared.

If you can only level one i would say maybe goldsmith since accessories are pretty hard to come by while leveling. DoH and DoL always sell pretty well.
But honestly it could be anybodies guess