Who is at fault?

If in fact you had the right of way, then the other driver is at fault even if he/she was at a stop sign.  He/She had a duty to make sure the way was clear before proceeding.

You'll hate this answer, but that depends.  If you have no stop sign and the other person has a stop sign, generally, yes, you have right of way and he would be at fault.  However, even if he had a stop sign, if he entered the intersection first and you were driving over the speed limit, you could be partially or totally at fault.  If there was a police report, I wonder what conclusion they came to.  The insurance companies may need to duke it out.  If you were not speeding, you have a better chance of being in the right.