Any and all tunes.
The worst ones are the tunes that can be endlessly cyclic.
My #1 on the all-time wormy list is "Storybook Love" by Mark Knopfler (theme from the Princess Bride). It just goes on and on and on (and up a fourth) and on and on (and back down a fourth again) and on ....
And when you don't know the words well and just fall back into the same verse over and over...
-Glenn