Its really up for debate... The common go-to pistol for Quietness is the TM Mk23 NBB. I've heard them in person and they're pretty much a whisper with a punch.
I've converted WE pistols into single action (You have to rack the slide for each shot) suppressed machines before. And the one i've made was much, much quieter than the MK23, but doesn't have the same velocity/punch behind it.
http://youtu.be/-sKZyPk668g?t=1m32s