The file
No melody, no lyrics, just a slap bass that seems to be making fun of you, and it's one of the most recognizable pieces of music of the decade. Jonathan Wolff built it around Jerry's monologue and changed it every week to fit the timing.
The bass stings between scenes were also his, and also different each time. It's a theme you can't hum and everyone can do.
Sources: Wikipedia — Seinfeld