I once lost a pair of gloves I had broken in perfectly during a snowy trip. It was ine of those pairs that fit my hands just right and I still miss them.
It's rare that I lose things, but I would like to have back a Garmin handheld GPS I lost that I got to use maybe 3 times, and a 15-rnd magazine full of Underwood hard cast for my SA 4.5 10MM..