Law & Order SVU
Peak viewership: Nov. 29, 2005, 17.54 million viewers
Lowest viewership: Jan. 15, 2014, 5.44 million viewers
When this show debuted back in 1999, it looked and felt a whole lot like the Law & Order and Homicide: Life On The Street episodes that preceded it. Along the way, showrunner Dick Wolf decided to let it be its own animal and let Mariska Hargitay, Chris Meloni, B.D. Wong, Dann Florek, Ice-T and Richard Belzer's unsinkable John Munch chew up as much scenery as possible, develop their characters and wring as much emotion as they could out of crimes that were already particularly heinous.
It took on its own life, but it also started looking a lot like CSI, Bones and other, techier procedurals that had the benefit of coming onto the scene without all of that '90s baggage. It tranformed into a completely different creature, but one that's more at home in its own skin in its most recent season that it was in its fast-quips-and-unsteady-cam days of the early 2000s.