None of them. I would get a solid education in a field that allows you riding time and a salary that pays for it all before I would open a stable or breed. The time and money that goes into a boarding barn or a breeding facility is shocking.
In my opinion I would say breeding. Although there are a few things to consider and vet bills are one major thing. Especially with mares and foals. If you want to get well known, you have to show that you put every penny into making sure the mare, stud and foal are all extremely healthy. With giving lessons and training horses there's the lawyer fees of making contracts. Flipping horses is probably the most easiest and cheapest thing to do. I would do a combination of two or three if you want to make a good amount of money.