Quoted from Deaconblooze:4MBC - I wanted an EM, and really like those games with zipper flippers AND multi-ball. I love that it has a skill shot, and some really great art.
Space Shuttle - For history's sake, quite an important game. Not a whole lot in terms of unique gameplay in my collection, though.
PinBot - Such a classic! I think it plays unlike most other games. Firing the ball into the chest area gives it a very random element. It's a great, all-encompassing theme. The lightshow is my favorite of any machine. It's very brutal in terms of ball times. Hungry outlanes make nudging a necessity.
WCS - I think this one is an interesting shooter. Really simple rules keep this fun for a lot of guests. I like the stryker shot, since it's the longest shot in my collection. A reachable wizard mode, and a good choice for competitions.
Heck, I'd be more than happy with any one (or all!) of these. Love your collection!