the vast majority of my games are opened, because I play my games. Sitting there sealed doesnt do them any good. I guess it would have to be Growlanser Deluxe, unless you think Disgaea, Nocturne, or DDS are actually rarer, but I dont think so.
I've actually got all 4 NIS PS2 Games sealed (La Pucelle, Disgaea, Phantom Brave, Makai Kingdom). I buy two copies of the game, copy it to the hard drive, and sell it off, leaving me with the sealed one and I can still play the game.