When you think of the worst songs of all time, what kind of music do you think of? What is great about that question is that bad music is subjective. Coldplay being a perfect example because some people adore them, and some people think they are elevator music with lyrics. Though we can say what we think on the subject, there is no right or wrong answer. But Metafilter put together a forum with that exact question, and some of these answers are pure gold.
Just keep in mind, you are going to see a few songs you secretly love on there. I saw "Lady in Red" and got mad for a second, and then remembered it was my parent's wedding song and they divorced, so maybe there is even some truth in the songs you don't want to see on there. Either way, bad music is mostly subjective (but some music is just plain bad).