Sci-fi Dine-in is the quietest restaurant at WDW.
I haven't been to a lot of the other restaurants mentioned but Sci-Fi is very quiet. It's why we eat there (quiet + dark and cool), and I have to say, the food in August was way better than we all expected. I think it's quiet because most people are paired up in cars so tough to talk, plus they're watching the movies. So...if you want to chat it might not be the best spot.