Ok, I personaly have so many of these, as I imagine most people do... but two stand out painfully in my mind.
The first is for strawberry flavoured mini-wheats. The song they sing, the horrendous cartoon animation, the dancing and the sound of that stupid talking wheat/human hybrid's voice. All of these combine to create a truly god-awful TV experience. The song drills right into your skull and stays there for weeks - driving you mad!
The second is for a product called "head-on" basically, a stick of some medicinal concoction that you rub your forhead. The ad itself is simple enough, with an anouncer yelling (three times repeated) "Head On! Apply directly to your forhead! Head On! Apply directly to your forhead!"
If either of these come on while I'm watching TV - it's a mad scramble to change the channel or hit the mute button, otherwise I've got crappy music and/or slogans stuck in my head for weeks and a renewed sence of anger to deal with!
I found the two on you tube so you can watch them if you like (at your own risk, I might add.)
http://www.youtube.com/watch?v=EGqlXBx_bBc - Mini Wheats
http://www.youtube.com/watch?v=Is3icfcbmbs - Head On
Oh and as a side note - I actually LOVE the shamWOW commercials! *guilty grin*