"This is the song that never ends,
Yes it goes on and on my friends,
Some people started singing it not knowing what it was,
And they'll continue singing it forever just because..."

Hey Ya has gotten a little old, but it's not nearly so archaic as this one!

The worst song to get stuck in your head, IMHO, is Bobby McFerrin's "Don't Worry be Happy."

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I am certain of nothing but the holiness of the heart's affections and the truth of imagination. What the imagination seizes as beauty must be truth - whether it existed before or not.
-John Keats