I have multiple votes. In addition to one instance while on security duty when a belligerent and armed (non-firearm) individual was held at gunpoint for local authorities.
I was the "victim" of an attempted car jacking. While driving to an out of town job early in the morning, around 4AM I stopped at a wayside to use the facilities. After shutting off the engine and pocketing the keys, I was approached by a wild looking individual. I closed and locked the driver door, and quickly locked the passenger door also. He proceeded to shout partially coherent obscenities at me, then moved around to the passenger side and tried the door. He then produced a folding knife and began beating at the glass. At this point he was looking down the business end of a Browning Hi-Power cocked, locked and ready to rock. Had he broken the glass, I would have fired. He ran off into the woods, and within seconds a Highway patrol entered the parking lot at high speed, lights off. Another motorist had been accosted and driven off, rolling the guy with the door and called 911. The guy was apprehended, in possession of methamphetamine paraphernalia. He was wanted, and later convicted of 2 nearby burglaries, including a brutal home invasion where he badly beat an elderly lady to make her give up valuables. When I was questioned, the county sheriff inferred that I should have fired. He also said thank God it was you he went after, not some soccer mom with a couple of kids in the SUV.
I have also used my sidearm to dispatch numerous nuisance critters and fired warning shots to chase off bears while camping.