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 stab

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.