My fixie is pretty quiet. Even when flipped to the freewheel, it's plenty quiet to sneak up on someone coasting.

My quiet test comes from riding through a pedestrian tunnel under I-10. In such a enclosed place, any chain noise bounces back to the ear. I think my quietest bike is my slick-tired, coaster-braked MTB. I can sneak up on someone in the tunnel on that bike.