At the moment it's Mr Brightside by the Killers before that it was the tune Children, but I'm always changing.
I have the ringtone of the ring of the phones in 24's CTU headquaters
How sad is that.
I currently have "Mardy Bum" - Arctic Monkeys, as mine.
Change it all the time, I think that the best ringtone that you could ever possibly have is the theme tune from "Jim'll Fix It". I had this for ages and only changed it because everyone else at work started to use it.
Also, used to have the 'ree ree ree' / shower scene noise from "Psycho" for when the wife rang