People keep saying to level all crafters, which feels to sort of defeat the purpose of behind the question "which is easiest to level"
The real answer is they're basically all identical (except for the gear slots they fill), and inter-dependent, meaning they really can't be levelled comfortably or cheaply without leveling each other.

Culinarian is the one odd-one out, and technically the only real answer to this question as far as I can tell, as it requires very very few mats from other crafters (just a very few alchemist crafts, iirc). If you just want to level one crafter to start with, without all the complexity of managing multiple jobs, and make use of any/all 3 of the gathering jobs to feed it, this is actually the easiest to level. It's what I did to keep a crafting job levelled while going thru MSQ and it was actually pretty nice for learning how it works without killing my inventory
The problem is you're left with a job that you can't do much with. You can meld your gear and craft consumables, which is really nice but can be very time-consuming and not nearly as profitable.