I think the only show I've ever watched that maintained a high quality throughout the entire run is the British version of The Office, and it was on for only 14 episodes, so that may not count. Breaking Bad did something that is very hard to pull off in that each season was better than the one previous. My problem with Breaking Bad is that it took me till season 3 before I thought it really became great. So maybe Breaking Bad jumped the shark backwards. Even shows that I think are among the greatest of all time, like the Wire, the Sopranos, BSG and Lost had runs where there was a drop in quality. As far as current shows that I love, Game of Thrones has maintained a high quality, but we'll see what happens in later seasons when they will have to start making things up. Justified seems to be on cruise control now and I'm starting to notice a dip in Sherlock.