In Guns, Germs, and Steel, using the chart on page 87, which of the five proximate factors is the most important product of an agricultural society?
In my opinion, the proximate factor that is the most important product of an agricultural society is epidemic diseases. The others are things that come from agriculture, but not so directly. You can see this fact in the chart itself.
The presence of epidemic diseases comes directly from the presence of many domesticated animal species in the society. Germs from the animals cross over to humans, causing epidemic diseases to arise.
The other proximate causes are important, but they are not so directly tied to agriculture. For those, there are many steps between agriculture and the proximate causes.