Do you own anything videogame related that NO ONE ELSE has? This doesn't necessarily have to be some rare and priceless artifact - it could also be something that was custom made or designed for you specifically.
Here are two examples, one from each category.
Atari 2600 8-in-1 Prototype - I can't actually guarantee that this is a one-of-a-kind item, but it's the only one known to exist (it's been in the CGE museum for three years now). This was to be a late-release 2600 game by Atari with 8 older titles on it. They did release a 32-in-1 in Europe which employs much the same technology.
Space Fury Guy Mask - You may have seen this in my game room or even at a Digital Press booth in the past. It's rarely worn and was originally created using the ColecoVision version of the alien commander from the Sega arcade game. It was designed by a professional mask maker in 2001 for the ten year anniversary of Digital Press.