I don't own any of these so it is just a 'maybe, one day' list for me.

Firepower - rewrote the rules of pinball play. Multiball, the outlane kicker and moving F-I-R-E with the flipper button, all in one game
ProFootball - even at 10c a game at the local arcade, I probably spent enough to buy a house
Kiss - the best balance of features I think I have ever seen, drop targets, bounce targets, the K-I-S-S matrix in the middle
Close Encounters - The first time I saw a roto target
Spiderman (1980) - How did they fit so much variety in one playfield
Target Alpha / Solar City- the drop target lovers dream machine
Meteor - The drop targets reset, then they dropped down again, magic
Hercules - The sheer novelty of playing something that big
Black Knight - If only I was coordinated enough to use the second set of flipper buttons for the magnetic capture thingy
Anything by Ed Krynski - so much imagination

Great thread