I didn't want to make a poll as the list would be too long as pops really have evolved over the years.
One of my favorite sounds is the sound of a pinball getting slapped silly in the popbumpers.
I love how every manufacturer's pops sound different, and they definitely changed/evolved over the years. Some motorheads can listen to a motor and tell you the manufacturer and even give an estimate to the year... some pinheads can listen to pop bumpers and tell you the same
I'm going to have to go with Williams early 80s (system 6, around there). Firepower's pops just sound so crisp and nice.
I also think the newer Sterns sound good.. absolutely hate their slingshots though.