Saw her doing I Kissed a Girl on Jools Holland last night and now the chorus is lodged firmly in my head going round and round and round and round and round. She can't sing live, incidentally.
Anyway, the worst one I ever had for this was Starman by Bowie which seemed to loop in my mind for a solid 4 months or so once and I still can't stand to listen to that song to this very day, despite it having once been a favourite of mine. A mention should also go to Maneater by Smelly Retardo which nearly sent me mental when I was doing my last set of exams. I would wake up at 4am and it would be halfway through the chorus (the bit where she goes "maneater make me blah blah blah") and I would feel like caving my head in with a rock.
Has anyone else ever been actually tormented by a tune?