4 games? Only?
I'm going to go with each "vintage" so you'll be ready to kick some Pin-butt at Pin-Burgh.
1) E.M.: Gottlieb Atlantis. There is no other game out there that has you shoot BETWEEN two drop targets to get a simul-drop for big points. Aim on the left, nudge on the right. This category is really difficult for me because there are just so many games from so many decades and countries of production to choose from.
2) Early SS: Bally Flash Gordon. Sports a ruleset that is so far ahead of it's time that these features are nowhere else to be found. And the fear. This design demands all of your attention as you can not hide anywhere on this game after the plunge. You will never grow bored of this design. Expect to have the game of your life against Ming... and then your next game will make you look silly.
3) 90's DMD: Bally/Williams Twilight Zone. The best. For so many reasons.
4) Modern DMD and forward: Stern AC/DC. I am SO temtped to put up other game designs. Some other games by other manufacturers are now showing their excellence, but AC/DC is the epitome of Steve Ritchie's craft and Lyman's code. That and the music is way sweet...
I could go on, but at this point, these 4 are very well balanced between skill, chaos and finally... flow. I hope to never have to make the choice of just 4. But these 4 are all brilliant in their own ways and are indicative of the best of their vintages.
Happy pin-hunting!
c2s