I don't know about my rarest game, like a lot of people here I just have some random 7s and 8s (on Coleco, Dreamcast and Neo Pocket mostly) with nothing really standing out.
Rarest system, though:
That's gotta be like R7 + R1 + R8 + R6 or something, for what? R21! ZOL!! OMG!!
Plus R7 more:
RAWR!! It's teh r@r3st sistem evar! R28!!
...word is bondage...