The Wilhelm Scream may be the most famous sound in history. I've never used it in a project, but I DO have a sound that I try to work into everything. It's my personal Wilhelm Scream. It's a mono recording called 'Suburban Neighborhood' that has birds, dog barks, and light traffic and wind ambience. Nothing special, but it's fun to work in somehow (often layered with other sounds) into nearly every project I work on.
And I know other sound designers use some sound or sounds often, mostly as an inside joke. So, my question is ... what's your Wilhelm Scream?
C'mon, 'fess up ...