Cows because they are easiest to fence. Though starting out with beef cow herd would take quite an investment and a long turn around of 2-4 years. However you could buy a few milk cows and buy baby dairy bulls and raise quite a few beefers for cheap. On the other hand beef cow mamas are about the hardiest domestic stock because of their huge rumens and fat reserves they can live off of very coarse forage through harsh conditions, especially certain breeds. You can also buy weaned beef bulls and steers and raise them and sell them very profitably. Cows are the least labor.
Pigs because they are the best at rapidly improving neglected land although they need lots of brought in feed so to have a large pig operation you need to be located somewhere where you can take advantage of a large nutrition stream. They need protein too which if you can't produce is costly. Natural pork is in the highest demand and pork production can be very profitable. It also can be the lowest startup, except not when you factor in the feed you will most likely have to buy. Depends on location and the environment.
Goats and sheep because they are the cheapest to raise and quickly reproduce allowing quick turn arounds on investment and ability to build up numbers quickly. However they are very at risk for parasite infestation and most farmers use poisonous anthelmintic wormers that aren't even very effective.
But pretty much cows or pigs is the answer. Unless you live in dry lands areas though then goats and sheep.