Mahler's 3rd. Not one of my favourites, but it is 1.5 hours long, so you would only have to listen to it 16 times. (OK, I know I am cheating - its supposed to be a song, not a symphony).
I don't know what song I would choose, but I do know that it will not be Celine Dions "heart will go on" song from the Titanic.
sing
bang yer head against the wall, till you get a sore
sore one of your shoes in half....two halfs make a whole
climb out through the whole
never be locked in a room ever again
Doesn't matter...with a 20month old kid, a wiggles song would worm their way into my head after 5 mins.