Gee - tough call.
3 games come to mind and id be hard pushed to choose between them, each for their own reasons.

NASCO Bingo bango 1951 - reverse flippers means you have to adjust your mind angle. You HAVE to lose at least 2 balls in order to get Super special, leaving 3 balls to build the game and score the special itself. But when she lights up - its beautiful!

Wms Metro - Disappearing jet bumper is the coolest feature ive ever seen on a pin. The short flippers on this make it a tough game - some really awkward shots. This games relatively new so novelty factor is still there. Can be a cruel game and also a very rewarding one - i keep coming back to it for the challenge.

But in by a nose is Bally Lost World. Had this a few years, love the artwork, the strong flippers. Also the kickout and spinner combo shot if timing is perfect. Every one loves this game. Seriously considering bringing this up from the games room to the front room. This game has proved itself over and over for me, is like an old friend