Mine would have to be WS after Illuminations when everyone is leaving. If you take the long way around it feels almost empty. It's so relaxing and nice; a good way to end the day.
I also love, for some odd reason, the area between M:S and Ellen at the old Wonders of Life pavilion. It's sad in a way, but I loved that particular area of the park, and it never has any people around it. It's a nice space to just relax and listen to the ambient music.