In People's Opinions...

Which of these 3 crafts is generally considered to be the cheapest, (best return for gil spent), easiest (ease of farming mats), and most profitable at 100+ skill....

Goldsmithing
Leathercraft
Smithing

Also which would best take advantage of alchemy sub? Just looking for the opinions of accomplished crafters because I'd like to start one of those 3 and can't decide on which. ^^

Leather, Leather, and Goldsmithing, in that order.

Leather is the cheapest craft and easiest to farm for.

Goldsmithing is the best for profit (Think "Spend 10M to make 20M")

1) Leather
2) Leather (I love farming so I love leather...)
3) its really up to you and how you play. Do you wanna make mats into other mats? Or do you wanna roll the dice and hope for HQ armor.

I have terrible luck so hoping for HQ probably isn't in my best interest lol. Making mats into other mats isn't bad imo.

Then i would take up alchemy. But out of those i would also agree leather is the cheapest to lvl, then smithing and finally goldsmithing. However smithing and goldsmithing i can imagine are more profitable when you get there.

Have you thought about Alchemy with sub smithing/goldsmithing?

leather.