Lol alchemy ever getting gil that's a good one Actually I make a lot of money off of alchemy but rarely sell big purchases, except for specialty items like augments, desynth books, and perfect pounces

Seriously though, while the greatest rewards of all of those may vary, and currently I'd probably say goldsmithing, what is most in demand changes with each patch so there's not really a consensus on what's "best". Though it's a safe bet to say half the time a patch will screw blacksmithing pretty hard, and the DoH classes tend to make the stuff you'd think they would make. It's also unwise to have a "main" DoH class. If you're only in it for the gil, you may want to stick to reading the market board for quick selling recipes worth a few bucks because endgame crafting outright requires if not strongly urges you to max out every DoH class, meld all your gear, and invest a lot of time, hard work, and gil to see marginal profits.

If you just want fast gil go spiritbond materia or gather. Good crafting is an investment.