The rarest game in my collection is a Capcom Breakshot. They only made 1,000 of them even though they seem to turn up fairly often. Mine is slightly more unique in that it is currently in the care of Jack Danger and makes appearances on the Dead Flip twitch stream.
The rest of my collection is mostly mass produced. Pinbot, Flash Gordon, Jungle Lord, Harlem Globetrotters, Gorgar. I guess Disco Fever might be thought of as rare. They made 6,000 of them but I do not think many survived.