A silent switch is the biggest factor in my experience. You can add o-rings to other switches to help reduce the sound of "bottoming out" (when the bottom of t

A silent switch is the biggest factor in my experience. You can add o-rings to other switches to help reduce the sound of "bottoming out" (when the bottom of the keycap touches the mounting plate or PCB depending on how the keyboard is constructed). But, I don't like the o-ring method because it's not as quiet and it also impacts the key travel, which doesn't always feel nice. Also, the o-ring only helps with the bottoming out sound, and some switches have an actuation point click that also makes sound and the o-ring doesn't change that. Additional foam, gaskets, etc can help with noise but it's pretty minimal compared to the switch itself.