Mad Men's first two seasons were really excellent, but I think the show's quality has irrecoverably declined since then. Two hugely important elements of the dramatic side of the story have disappeared over time (I won't mention them), and now I think Mad Men is basically self-parody and one-liners. That doesn't mean it still isn't fun to watch, and occasionally hilarious, but to be the most celebrated American show in history (as measured by Emmy nods) is quite the stretch.
I only saw the first two seasons of Shameless (UK), but they were another slow-burning treat.