I'll always say 'The Wire' is the best show in the history of television. Not just for entertainment value, but the statement it makes on poverty in America in the 21st century. It's as much a sociological presentation as it is a crime drama. It's stunning, riveting, depressing and perfect.
That said, this past month I've gone back for the first time in a half decade and re-watched 'The Shield'. The first 3 seasons are as close to perfection as any other show has gotten save maybe 'The Sopranos'. It's not rooted as deeply in the psychological reasons behind (and damage caused by) crime, but in portraying the honest and dark picture of street life it comes close. Although it fell off the rails a bit starting in season 4 and never really recovers, it still earns a place among the greatest TV shows of all time.
I don't think there's any question the last 15 years are truly the golden age of television. 'The Wire', 'The Sopranos', 'The Shield', 'Breaking Bad', 'Sons of Anarchy', 'Homeland'. I defy anyone to think of any other time were as many amazing and groundbreaking shows were all being produced more or less at the same time.