Pinball circus at the Hall of fame. I'm not sure if it counts, but the rarest one would be the one-off Tumbleweed game that is currently at Pizza n Pinball in Newcastle Australia. A local guy built it from scratch and themed it after his favourite band and it is currently on site.
http://www.aussiearcade.com.au/showthread.php/62755-Tumbleweed-Pinball