In deciding the high point in pump guns you have to divide the contenders into First Generation guns and Second Generation.
The First Generation guns were designed from the 1890's up to WWII.
These First Gen guns were pretty much all forged and milled steel guns, and were tanks that just never wore out.
There are verified cases of Winchester Model 12's used as aerial training guns for pilots and aircrew that fired 500,000 rounds during WWII.
Of the First Gen guns, the Winchester Model 12 is probably the top due to the quality of steels used, and the high end workmanship. It's that quality that edges the other First Gen guns out.
Other guns like the Ithaca Model 37 were great guns, but Winchester was just a step above in quality.
The Second Generation guns were designed from the end of WWII to the 1990's.
Of these, the Remington 870 is the winner, for much the same reason the Model 12 was the top gun of it's era. Remington simply built a better quality gun from tougher parts.
The 870 got it right by using a forged and milled steel receiver with heavy-duty fabricated internals.
Remington's design was and still is one of the simplest designs and uses stronger internal parts.
Some of the very high round count clay pigeon shooters say the 870 starts to develop cracks in the receiver somewhere around 250,000 rounds.
The police have used the 870 almost exclusively since the 1950's and it has an unbeatable reputation for durability and quality.
The other Second Gen pumps almost always used cheaper to make aluminum receivers with stamped internals, and some plastics. Internal designs are usually more complicated, and use smaller parts.
According to the clay shooters, these aluminum guns start breaking at around the 50,000 to 60,000 round mark.
The Third Generation guns are the plastic guns that have started appearing.
These are all too new to indicate a clear winner.
Come back in 20 years for the verdict on these.
An oldie but a goodie, a model 1897 original riot takedown made in 1940. 
