My wife is a music nerd, so I asked her this question. Her answer: Summer Nights from "Grease". The entire song is perfect for what it is, except for the very last word. John Travolta switches to his falsetto range to sing the word "nights" in a style that reminds me of the Bee Gees, and it just doesn't fit with all the vocals that came before it. It's literally a sour note to end what's otherwise a perfectly good song.