I've been hearing for a long time that Chili's "Baby Back Ribs" jingle is number 2 on the list of the stickiest songs, but I didn't know what the number 1 was. Here's this list. It was compiled in 2003 by a marketing professor James J. Kellaris, PhD, of the University of Cincinnati. His sample included 559 university students.
1. Other. Everyone has his or her own worst earworm.
2. Chili's "Baby Back Ribs" jingle.
3. "Who Let the Dogs Out"
4. "We Will Rock You"
5. Kit-Kat candy-bar jingle ("Gimme a Break ...")
6. "Mission Impossible" theme
8. "Whoomp, There It Is"
9. "The Lion Sleeps Tonight"
10. "It's a Small World After All"
BBC had a follow-up article explaining the entire earworm phenomenon.
A good book on the subject seems to be This Is Your Brain on Music: The Science of a Human Obsession. (Reviews: Salon, Wired)