Godzilla Pro - $7000 - It is the current GOAT imho and you can get it NIB or used easily. Mr Elwin humbled all of us.
Star Trek Pro - $5,000ish - Like ST or not, Steve Ritchie nailed it here - this game is fast, easy to understand and will actually make you think you are getting good at pinball - "wow, I didn't even see I made that shot... but I did - wow I must be getting good." LOL!
Pinbot - $3000 - Python and Barry masterpiece and still the best light show in pinball. Come on - 2 sequels, an NES cartridge, he makes a cameo in Taxi and it is a fairly easy teardown for shopping.
Firepower - $1500ish - Damn Steve Ritchie! Again, a masterpiece and the first SS multiball. This thing has the best sound (which then is sampled in Deadpool decades later!) and you will find yourself obsessed trying to get multiball - at Pinball HOF they have one with drops... I stayed until they closed and told me "last game guy!". What a great layout dammit.
Grand Prix - $1000 - One of Williams last EMs before moving to SS. It is fast for an EM and affordable. Literally smash the crap out of 2 spinners and enjoy!
