Without question, the quietest router is the one that isn't running. ":^)
If you don't mind, please share why you need to know which is "quietest".
They are all loud enough that common sense dictates the use of protection. If you are worried about disturbing the neighbors, the db level between routers isn't big enough to make a difference. Table mounting will give the best results for lowering the sound level. (Table design can make a huge difference.)
The choice should be based upon your routing needs, not the db level.
You think I'm crazy? Wait till you meet my imaginary friends.