The worst song ever is undoubtedly 'Brennan On The Moor' which I had the misfortune to hear many years ago performed by a guy in the stereotypical garb of Arran sweater, pint pot in hand (actually one of the very few people I've seen so attired), with much enthusiasm and little talent. This geezer was so delighted with his own performance that he repeated the damn song.
It seemed like poetic justice when, many years later, Father Ted kicked Bishop Brennan up the arse.