Closest I ever came to a mid-air was probably a couple hundred feet. I was up with a CFII student in a 172, in the practice area north of Daytona.

Student was under the hood just practicing a little BAI at 4,700 feet. I picked up some motion in the very upper left corner of the windscreen, and saw a Seminole pop out from behind the air vent. They were slightly above us moving left to right and descending on a collision course, and were close enough I could tell that the right engine was feathered and the student was under the hood in the Seminole.