My pick for a one pin collection would go to The Shadow, due to balanced scoring and interesting rule set.
The thing I like about Shadow is that there are many ways to score points, both on the way to the final battle (wizard mode) and just in normal gameplay (orbits, shadow loops, vengeance, super vengeance, battle for Kahn, Mongols, up to 7x combos, a secret laugh mode), the multiballs (if well played) can bring in big points, especially if you are able to lock the ball for double/triple jackpots, and hit the super jackpot for 100MM (then x2 or x3); there are four different multiballs in this pin.
The game is very well thought out, with many nuances, like additional points for hitting all the targets when defeating Khan, or following a Shadow loop from an orbit. There are many ways to play this table to get lots of points, and one doesn't dominate. Someone achieving final battle, can be beaten by someone dominating the vengeance mode (hitting rings and points, and this person can be beaten by someone playing shadow loops, and this person can be beaten by someone ELSE who kicks ass at final battle (it's a paper scissors rock loop). The game is very balanced and has that "one more game" factor.
In terms gameplay and design...the game boasts some of the best audio in pinball, a stylish, classy art deco design, 5 ball multiball if you can get to wizard mode, gold wireforms, ramps that you can control what flipper they arrive to making the game more interactive, a third flipper with 3-4 possible shots (always a plus in my book), a breakout style mini playfield (IMO even better than TZ's) and one of the coolest lock tricks around. The downside would be the backglass (but there is an alternate translite option). This is Brian Eddy at his best, making old school pinball, without having a central toy dominate the rule set (i.e. AFM and MM - both amazing games too).
I have a one pin collection too, guess what pin I own?