My rarest would be my copy of Daytona USA CCE: Netlink Edition. Was a bitch to find to finish my complete Saturn Collection. Also, my Sonic Adventure Limited Edition to finish my Dreamcast collection.
My most treasured? Would have to be my Dragon Force... I still remember when I bought it new at ToysRus(which was the main place to buy games in Canada during that time as the Saturn wasn't very well supported then, or ever here)