I am just beginning in Colecovision collecting, so I don't know what would be my rarest, but I know some which may be hard to find..
I have about all of the Imagic and Xonox games complete and mint. Wizard of Id's Wiz Math complete in box might be the one I have which I consistently see rated the rarest. I also have all complete Spectravision games like Sector Alpha, Flipper Slipper, and Frantic Freddy. Up n Down, Threshold, and some Sierra Online complete as well. I was told one of my B.C.'s Quest games was a box variation, don't remember which. I've got some other stuff which seems great to me, but like I said, I don't have extensive knowledge on rarity, so I will end it here.