A bit sad that no one mentions MARDI GRAS from 1962. maybe because this one is rather unknown in the USA but believe me it was a 'Killer' over here in Germany. It was the first 4-Player with a production-run over 1,000 - of course most of them for Germany and Austria while France was totally dominated by Gottlieb's 4-Players.
But the question goes for the 'signature game' and I think this would be SPACE MISSION / ODYSSEY. Close to 16.000 units speaks for itself. Steve Kordek was very proud of it and had a Backglass hung in his office until he left.