Here are some nominees for worst song ever. These songs are horrible. One is emblematic of the end of a star's best years. One has just horrible lyrics ("Someone left the cake out in the rain/I don't think that I can take it/'Cause it took so long to bake it".) One is a horrendous version of an otherwise fine song.
Party All the Time
Mr. Tambourine Man (as performed by William Shatner)
Honestly, just doing the research on this one was painful.