The Wire. It doesn't treat the viewer like an idiot. It'll ruin other series for you.
Boss. Only ran 2 seasons on Starz; easy binge. Super-intense Kelsey Grammer as the "fuck you all" mayor of Chicago, and includes the super-easy-on-the-eyes Kathleen Robertson. Incredible.
And as mentioned, Deadwood is spectacular.
And I love Homeland, despite hoping in every episode since Season 2 for Claire Danes to catch a bullet.