Why does keeping the receipt count for so much? I don't get it other than proving to the buyer that the game is legit and not a copy?
Anyhoo, I don't know which game I still have would be the most rare. At one point I had Alice Interactive, Treasure Hunter, and Synnergist in my possession, but I sold them. Heh, my proudest moment actually was ripping off the shrinkwrap of Alice so I could play the darn thing.
I'm sure that did the resale value no favors, but I am a player first.
I do still have a boxed Hamlet, U.F.O.s, Celtica...I think those are still considered rare. I also have all of the classic Sega Saturn RPG's which are somewhat rare.