The worst song has to be "MacArthur Park". A lot of other really bad songs have been parodied succesfully so that you can sing mildly amusing lyrics instead of horrible ones. I'd suggest trying to create alternate words to any tune you're stuck with. It could help.
On a lighter note, I've had an Irish hymn stuck in my head all day, "Morning has broken". A lot better than "One-ton tomato" or whatever that song was. OR anything William Shatner has sung. Or "Dawning of the Age of Aquarius" or "Feelings" or...
do you hate me yet?