In my opinion this is no contest. The worst song ever, along with everything about its performance, sentiment and accompaniment is Toni Fisher singing "West of the Wall". This was a hit in 1962 and I can remember hating it at the time and the loathing stays with me still. At the time this smug American propaganda was being peddled as entertainment, the CIA were doing their secret best to undermine democracy in countries in four continents.
The lyrics are trite lies, the melody has nought to recommend it apart from its ability to become an irritating earworm.
Then there is the performance.... The lead singer's irritatatingly bothersome quality is not helped by the fact she does not always hit the note full on, the accompaniment is trite in the extreme and just when you think it cannot get any worse, we get to the middle eight which is handled by the declamatory grumbling male ensemble.
Nothing, no nothing in music can ever be so bad again,