One of the quietest I have even experienced is my 1936 Corona Standard portable typewriter. To me it seems considerably quieter than my other regular typebar typewriters. Yet, it has no insulation or felt on the inside of the machine.

I assume you're talking about the quietness of the typing action only? Some machines make the "zzzz" sound when you return the carriage, of course, and some have a more rattly space bar than others. (The Olympias are the only ones that have a silent carriage return, in my experience.)

Quietness of typing can depend a lot on the hardness of the platen. Royal portables seem to have remarkably pliable platens, even 60+ years after manufacture. (I am assuming that the vast majority of portables have their original platen rubber -- they don't get enough use to warrant platen re-covering, and that's an expense their owners would have never been willing to bear.) So I'd have to say that the Quiet De Luxe models I've owned, and similar models like the Arrow, are pretty darn quiet, not just because of the platen but because they seem to work well with a pretty light touch. In fact, I think these machines "crowd" characters partly because you can type with very little effort, and end up going too fast for the mechanism.

I also have a 1968 Olympia SG3, which in addition to having a really nice and consistent touch, types very quietly. I am sure the size and mass of the machine is a significant factor.

I'm interested to try one of the Remington Noiseless machines, to see if indeed they are quieter than the norm.

Also FWIW my Erika model 8 has a silent carriage return as well...

The sound quality and noise level of a carriage being returned is influenced by the condition and adjustment of its associated parts. Most carriages should be fairly silent in their operation, the predominant sound coming from the bearings that its gliding on. If you have a carriage that is very loud, most likely it's in need of either cleaning, lubrication, adjustment, or repair.

​I can't agree with the conclusion that Royal platens stand the test of time better than those of any other make. Looking over the hardness tests of a few of my Royal portables ranging from a '48 Quiet De Luxe to a '61 Futura 800, they all have hard platens (between 97-99 Shore A). This is in line with the results of testing platens from other manufacturers. If you have a 60-year-old Royal portable with a soft platen, I'd be willing to bet that it was recovered sometime during its more recent history. Keep in mind that typewriters were an investment in their day, even portable models, and those who used them a lot would have been likely to have them serviced, much like you would your car, and regular or professional users would have had their platens maintained too.

​I'll be honest and admit that I have no idea which of my typewriters is the quietest. I did test a few using a decibel meter, but that was just part of an experiment to determine if the "quiet" portables were actually less noisy than their regular portable counterparts. I type in a number of different locations, which makes it difficult to provide a fair noise comparison between numerous models. And then there are so many factors that influence a person's perception of how 'loud' a machine is such as ambient noise, the surface the machine is on, the condition of its feet, its overall mechanical condition, even the typing technique being employed.

If I had to guess, I would probably nominate my Underwood Noiseless 77 and Remington Noiseless Model 8, along with a few of the thrust-action models too. I would hope that the quietest typewriters are those that were actually marketed as being more quiet than regular models, ones that included words such as "quiet", "silent", or "noiseless" in their model name.