In Guns, Germs, and Steel, which animals most helped Eurasians in Chapter 9?
Jared Diamond lists fourteen different animals which he argues most helped Eurasians. He groups them together as the "Ancient Fourteen." Interestingly, all but one apparently originated in Eurasia; yet all had distant relatives (similar animals) in other countries who were not so easily domesticated. He also draws an important distinction between "domesticated" animals and "tame" animals Domesticated animals have lived with and in relationship with humans for so long that they have evolved by nature to function well in captivity. "Tame" animals are not so hardwired. Diamond says animals which could be domesticated had to be gentle and passive enough not to be a danger to the humans who came to depend on them. He also comments that the animals normally were gregarious "pack" animals who were accustomed to following a stronger leader. This, he argues, allowed humans to step into the role of the leader which the animals would follow. This is certainly true of horses, pigs, goats, cattle and sheep. Dogs and other animals were domesticated by humans, but did not provide the enormous benefits that were provided by the Ancient Fourteen.
Aside from furnishing meat, the Ancient Fourteen were work animals who could pull a cart or plow, give milk, etc. This made them immensely important to the Aryan peoples of Eurasia.
That would most likely be the horse, which the used to fight wars and gain victories with the Americas. Another option would of course be the cow, which has value for it's meat, or the ox, which makes a good draft animal, but I believe that the horse was the most helpful to them.
I think your first need to define what "helped" means. Does it mean a better way of life, advancement, survival, ward of enemies, conquer lands, health, enlightenment, etc.? Even if you do narrow it down, I think there would still be a subjective view, though. What might help one person or people, might be a detriment to another.