For many years, I've put the initials of users and the month and year on the waistbands of all the cumstained underwear and jockstraps I've traded or lent out to other guys. Some of them have been used by a dozen or more guys. Love jacking off with them.
Hard to pick, but I have a vintage Bike M jockstrap that I've had for many years, and have loaned to 20 guys who all added their sweat and sperm. Here's a pic after being used by just a few of them. It's much more stained now and smells great!