For me, it's touch. I was teaching anatomy and physiology in a massage school a few years back. I told the students that their sense of touch would develop to the point where they could feel a human hair under a piece of notebook paper. One of the students wanted to test me. I said that it was a trick, because there were 3 hairs under the paper. They were all surprised.
From the age of 13, through undergrad, I worked with veterinarians. I could find fleas on an animal, just by petting it.