How can you determine which animal has the best hearing?

The trick with testing animals for their hearing is that they do not necessarily want to tell you what they do or do not hear, so you have to create some reason for them to react to faint sounds. You could, for example, build a large series of buttons, designed so that you can have any one of them emit a sound, of any desired loudness, and if that button is pressed when it makes a sound, the animal is rewarded with food. (Not all animals can press buttons, but equivalent mechanisms can be devised that would meet the requirements of other animals.) Then, when the animal is trained to respond to sound, you can find out how faint the sound can be while still getting the animal to respond. Then, if you test every species of animal, you can compare results and find out which animal has the best hearing.