I know it is a common or expected response, but Game of Thrones is the most entertaining thing I've ever watched. The plot is just so damn big, and there are so many characters and so much history.
Black Mirror, however, is simply amazing. With Black Mirror each episode is a 'stand alone' episode, i.e., a tale all of its own. And this stuff is some high tech Twilight Zone in the modern day. I've watched all of it available, and I'm already dying for some more.