Of all the songs from the soundtrack of O Brother, Where Art Thou? to get stuck in my head, it has to be the one with three little girls singing shrilly ad nauseam:
In the highways, in the hedges,
In the highways, in the hedges,
In the highways, in the hedges,
I'll be somewhere a-workin' for my Lord...
I'll be somewhere a-workin',
I'll be somewhere a-workin',
I'll be somewhere a-workin' for my Lord.
I think this one beats out It's a Small World After All, Music Box Dancer and The Wheels on the Bus Go Round and Round for worst song to be stuck in your head. Even Spanish Flea can't drive it away, and that one usually does the trick.