All of them.

Crafting in this game has a lot of overlap between classes. They all use the same armor (Barring some JSE options at 50, 60, and 70), there are cross-class actions where you can use actions gained on one crafter on a different one, and there are many secondary materials made by other crafters that are required for recipes. Then you toss in how you'll need a fair amount of classes to make any job's gear, toss in food and making gear for the crafting classes themselves and... yeah. Best to just do them all at once.

You may also want to level up gatherers alongside them as well, so you can gather your own materials as well.