@Jammer, I am not sure if there exists such a rating that you are looking for. My personal view would be ,make a study of your requirements-whether you are looking for a certification or a compliance or just trying to increase security. Based on these criteria,you can look at the pentesting organisations and evaluate them on your own. This link may help,
http://www.ivizsecurity.com/blog/penetration-testing/how-to-choose-penetration-testing-companies/
Anyways there is always a trade off between choosing third party vendors or owning a own security team. You can go for third party consultation then have a own in-house Security Educated QA Team.
Hope this helps.