Silent linear switches are the quietest. If that's not an option, then linear switches with o-rings.

Also you frequently get quite a lot of noise from the stabilizers on the large keys. Your only real option there is to lube them, but that's not really feasible on most off the shelf keyboards.

Quiet as possible: Reds (any company, I prefer Cherry).

Quiet enough that your coworkers won't hear it, yet very comfortable to type on for those new to mechanical keyboards: Browns.

Reds + orings, or brown + orings. A lot of the sound comes from someone bottoming out the keys.

I seriously discourage the use of orings. Rather, new users should practice _not_ bottoming out, which is why I recommend Browns for new users.

What’s the reasoning against o-rings?

Orings reduce the total travel, probably by about 6-8mm judging by their thickness though I've not measured it. Traditional Cherries would register at about 25mm and bottom out at about 40mm. So the user would have to train to depress to at least 30 but no more than 35 mm depth. That takes practice, and practice requires making mistakes. The orings bring the bottom of the stroke up into the range where the user needs to practice releasing the key on his own. If you're bottoming out on a mechanical keyboard, you are negating the real benefit of the device.

I've not used the low-profile switches, but I believe that they activate at around 12mm and also bottom out at a lesser depth though I don't know where.

This is why I highly recommend browns (any manufacturer) for new mechanical keyboard users. It is much easier to train on the tactile browns. You could do it with blues as well, but blues are just too noisy for office work, and gamers say that they have too long a reset stroke to be good for gaming. Once the user gets used to not bottoming out on browns, moving to a Cherry red keyboard is absolutely amazing. Typing on reds feels like floating on a cloud, it becomes an enjoyable experience in its own right, once you learn to effortlessly not bottom out.

This seems an awful lot like you demanding users have a particular skill you want them to have for $reasons. This is silly, use orings.

If the user is bottoming out on orings, then he might as well use a rubber dome keyboard. There is almost no benefit to using a mechanical keyboard if the user is bottoming out.

There are some terrific rubber dome keyboards. I actually use one occasionally as I like to rotate my equipment, to fend off the R in RSI.

I did not do tons of research but I ended up with Cherry MX Clears.