Potato

Some standard potatos.

The potato (aka spud) is the original and most common ammo for a spud cannon. Typically large and juicy ones are perferred, this is because the size is needed for larger barrels and the juice acts as a lubricant in the barrel when muzzle loaded. Also when muzzle loaded, the spud is typically forced into the barrel that has a muzzle knife on the end. When this is done, the spud is shaved to a perfect fit for the barrel which makes an air tight seal.

Potatos are also typically fairly cheap and can be found anywhere, thus why it started as the original ammo.