60 in 1.
Ms. Pac-Man
Neo Geo
Pole Position
Space Invaders
The 60 in 1 is easily the most common game out there now. They probably surpassed the original Ms. Pac-Man production run 5 years ago. I would not be at all surprised if there were a million or more of them out there now.
Always see Ms. Pac, never see Pac-Man.
I think we all underestimate how many Neo Geos are out there because so many of them are snapped up by console game collectors and never return to our world.
I have only owned 5 Pole Positions myself. However once when a bunch of us got to loot a warehouse for free, at the end of the day I was the last one there, and there was nothing left but 2 heavily water damaged cabinets, and something like 5 Pole positions. I hung around taking parts off the Poles until I ran out of time.
Space invaders was high production, almost never converted, still seems to crop up a whole lot, particularly if you count the clones and variants.
Defender, Joust and Stargate would belong on this list but there are a bunch of guys who hoard multiples of the Williams games, making them less available than they should be.