EC-120
EC-135
MD-900
MD-520N
Are all pretty quiet. From reading about the subject I´mled to believe that the MD-900 is a bit quieter than the EC-135 when using the US mode of measuring overflight noise at 1000´ with severeal sensors.
You mention "heavy horses"? Did you mean in the SA-365 & S-76 class?
I would think that the EC-155 would be the winner as far as noise is concerned in that class, but that´s just a guess!