For about the first 10 years of my collection, all I had was Early SS Williams games. There are soo many great games from the era it is really hard to pick just one...hence why my collections just seems to keep growing. I can't seem to give any of them up! You have System 6 games like Firepower, Blackout, Scorpion, Algar, Gorgar... Then the system7s Black Knight, Solar Fire, Barracora, Warlok, Time Fantasy, Cosmic Gunfight, Star Light (I could name them all but you get where I am going!) System 9s (a bit easier cause there is only 3 basically) Space Shuttle, Sorcerer, Comet. System 11s like High Speed, Pin*Bot, F-14 TomCat, Taxi...(again lots of choices here) Early Ballys, I am not as familiar with, but Fathom is a Great game! (Really need to get mine back together to take to some shows...) Stern's Star Gazer is one of my favorites...I guess it all just comes down to what you like...My suggestion would be go to some shows (if you are near Pittsburgh try to hit RePlayFX coming up the end of July, the PAPA facility has around 400 games to try so you could get a feel for what you like and would want to add to your collection) or find some collectors in your area and play some of the games you are thinking of, then when you find one that fits your qualifications then the hunt begins...
Good Luck with your hunt!
Phoebe