Most annoying song, period.

I’ve got my mind set mind you - George Harrison. The most banal lyrics plus he repeats the title 24 times. This always catapults me out of the chair to change stations.Also, unfortunately, any Adel song, as was trapped for two months in an office where this looped constantly every 90 minutes, and I started a fan and resigned to get away from it. Now her music seems to me to be her moaning all the time about lost love.

Gwen Stefani "Hollaback Girl".

"Miss. Gradenko" The Police

 (I think I win.)

Anything by Gino Vannelli. If you’ve never heard him, consider yourself fortunate. He took pretentiousness and affected vocals to new heights in the 1970’s. Listening to his music can actually make me want to throw a cannoli at my speakers!.
As far as the worst song that was ever a big hit, my vote goes to Having My Baby by Paul Anka.
Some of the other most hated songs on this thread such as Macarena and Achy Breaky Heart suffered more from the resentment caused by their overexposure since those songs played endlessly everywhere you would go
for months on end and it drove people nervous (as Alice Copper said).

Oh holy cow - you guys have come up with so many of my least favorites - well done! I may have overlooked these two, so forgive me if I'm duplicating...

The Carpenters - We've only just begun *cringe*  (sorry, Karen C.)

...and...oh good Lord...I almost can't make myself type this one...

Maria Muldaur - Midnight at the Oasis....NOOOOOOOOO!!!