Top PR Companies are often either identified by size, or by their achievements which often bring industry awards. In the UK, the Public Relations Consultants Association which represents the top PR companies, and it’s members area has listings by areas.
PR Week, the public relations weekly paper also has a catalogue of all existing PR companies in the UK, and provides ranking information for them also.
SABRE (Superior Achievement in Branding and Reputation), and PRWeek also give annual awards to the top PR companies under a host of categories.