Easy! Someone Like You (Adele). It was like my favourite song for aaages a couple of years ago, but it just got overplayed. I can't exactly hate it- partly 'cause, as I've said, it's still good, and partly 'cause they're are a lot of memories attached to it. However, it definitely went down a lot in my opinion just due to the constant replaying.
Someone Like You: http://www.youtube.com/watch?v=hLQl3WQQoQ0