Whatever the standard Blackberry ring tone is. I couldn't be bothered to change it. Same with texts too. I'm not that bothered about ring tones, as long as I can hear it. If I could be bothered though, I would have either L's or Near's theme from Death note as they strike me as being the perfect ringtones.
I do prefer Near's theme to L's though.
I do prefer Near's theme to L's though.