I not-so-proudly present the worst cover that I have ever had the displeasure of hearing...
Avril Lavigne - Chop Suey
http://youtube.com/watch?v=twvdypF4pUg
The original is not a great song by any means...but there's no way in hell it deserves this treatment.
And here's a fun one...
Snoop Dogg - Sad But True
http://youtube.com/watch?v=gKHarz6BUDk