Which building is the best building to upgrade and spend money on to get the most money?
The earnings depend on the level. You will have to upgrade them to a point and see for yourself based on the current season. It is difficult to give a blanket answer. Spend upgrades on a particular building and it will become worth more in general. For events in particular, the most valuable building seems like a random roll for the sole purpose that people will not know the best buildings in advance.