Both companies made some excellent machines, and their merger only made them stronger.
But, I'm curious, who was your "money-maker" of the duo?
I cannot deny that Williams had some fantastic and persisting ideas. Games like Taxi, Medieval Madness, Funhouse, the Pinbot series, and High Speed still live on the the hearts of many to this day. Star Trek: The Next Generation is getting the most playtime out of me, personally.
However, in my opinion, Bally was the true king of Pinball. The Addams Family was one of the top sellers of all time, and even got me into playing pinball. The Twilight Zone is one of the most beloved machines ever created. Attack from Mars, Theater of Magic, and Safecracker are also highly regarded.
So, I must ask; in your opinion, which of these two were the powerhouse behind the Bally/Williams brand?