One of the toughest machines that I worked to find was a Bally Vampire. These early Bally's were not the most durable, so many ops would often part out machines to keep others on route, leaving very few operable pins still available.
I love the campy graphics, the center ball save post activated by rollovers, and old school chimes.
Fun theme and a nice casually player for non-pin head friends.
cheers,
rody