I guess technically my rarest item is a 1 of 1 Destiny art print from that art show they did before release, along with the other ones which were limited to around 20 each. The art itself isn't too special, but the CoA is top notch!
Other than that, my rarest items are probably from my GTA collection - the Kubrick triple pack, GTA III 10th anniversary engraved bat among a lot of smaller items.