I just try to look for games that have a good combination of several factors.
-It has to be challenging. Easy games get old fast.
-The rules or gameplay should not feel overly repetitive
-mutiple objectives or ways of scoring is always nice
-killer theme, artwork, music/sounds. These always help
FunHouse is a good combination of challenge and theme, but it can get repetitive... as an example. Bottom line though, the older the game, generally the more basic the rules will be. This is okay for some early solid state pins because they make up for it with very difficult gameplay, but that isn't always the case.
My personal tastes would included getting at least:
-1-2 good early 70's EMs (think stuff like El Dorado or any of it's clones, but there are so many good ones from this era. Get something with lots of drop targets)
-a couple of early 80's solid state games (classic Stern like Big Game, F2K, Seawitch, Stars, Meteor etc, or classic Bally like Eight Ball, Centaur or Fathom ... if you can find one)
-System 7-11 games like Space Shuttle, High Speed, Pinbot, Taxi, Swords of Fury are not super deep, but great themes and fun gameplay.
-For DMD you can't go wrong with stuff like Twilight Zone, Shadow, BSD, White Water or Medieval Madness from the 90's era, or newer Stern games like Tron, AC/DC, Metallica etc.
That should be a good list to start with.
Watch the online tutorials for reference. Just be warned that those guys typically just play towards a specific scoring strategy, so some of those vids can make an otherwise fun pin look quite boring.
Hope that helps
