I'm going to throw Gottlieb's 1950 Bank-a-ball in here. Played it last year at CAX and I kept coming back to that game. Its simplicity is mindblowing, yet completing is is something very difficult. Note that there's nothing on the playfield. No bumpers, it's all wide open. Simply hit all the 15 balls as they are aligned on the sides, with the 8-ball being a tough one at the center top. If I can find one I may have to buy it.