It's the BAe 146/Avro RJ series. They are the quietest from the outside but not the inside. I believe the quietest from the inside is indeed the 717.
The BAe 146/Avro RJ series is nicknamed "The Whisper Jet" because it is so quiet. When a 737 or any other jet takes-off and then a BAe 146/Avro RJ takes off right after. You can really tell the huge difference between them.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
I would have to say the 717 or Avro/Bae 146/MD90 from the outide. I agree with High_flyr69 the A320 family isnt that quiet from the outside, they make a wierd noise on approach and takeoff, if you know aircraft you know its one of those.
The fact the BR715 is such a quiet engine is all the more reason why AA ought to seriously look at installing an uprated version of this engine (along with a quiet engine nacelle) as a replacement for the JT8D-217/219 engines installed on AA's current MD-80 series fleet. That way, AA could keep their MD-80 series fleet flying until at least 2017 even with stricter noise and exhaust emission regulations in place.