I think one of the best (if not the best) is 'Prime Suspect' starring Helen Mirren.
Mirren plays ballsy DCI Jane Tennison who rises through the ranks of a male dominated, sexist and corrupt 90's Metropolitan police force. Prime Suspect was voted 68th in the list of 100 Greatest British Television Programmes in a poll by the British Film Institute.
Another ITV show that gained excellent viewing figures and critical reception is Broadchurch, starring David Tennant and Olivia Colman searching for the killer of a young boy in a small coastal town.
My final suggestion is The Killing (Forbrydelsen) which was screened on BBC4. The series focuses on one murder, and is twenty episodes long, which builds up amazing suspense! It is a Danish TV show and has been made into an American version, but I would recommend going with the Danish one: