Bally Fireball from 1972 would have to be considered the most sought after Electro Mechanical pinball machine from the 1970's, and for good reason. There's just so much going on with the Zipper Flippers, spinning playfield disk, 3 ball Multiball, tough as nails skill shot, and the amazing art package. I am surprised that they only made a little under 4,000 of these, but I figure it was considered a success at the time because they eventually followed up with a Fireball Home Model at the beginning of the Solid State era. Then there was Fireball II in 1981, and finally a remake of the original with Fireball Classic from 1985.
The EM version is a great party game. Ball times are relatively short, and the way the multi ball system works, it's possible to walk up to the machine in a multiplayer game and have the previous player do all the work for you by locking the first 2 balls ready for you to release them.
I only have one minor issue with the game, and that would be the randomness of the spinning disk. But I figure the game wouldn't be the same without it. To me it just adds a "more luck than skill" element to the gameplay, but man is it fun!
So obviously as this game is sought after, it commands a high price. Personally I am a fan of the Solid State Classic version. They are much more obtainable, and is assentially the same game but without the Zipper Flippers. However it does have cool early 80's sounds, 7 digit scoring, and an end of ball bonus system.