I was pondering a topic along this line just a few days back. My query was more along the line of which year had the most dramatic changes, features and uniqueness. I personally would be hard pressed to go against 1971. The B-body was redesigned with a body that had no predecessor. The Rallye gauge cluster was a huge improvement on previous styles. Both Plymouth and Dodge were almost 100% unique of previous versions. I'm a Plymouth guy here. The Challenger in 1971 took the features of the 70 and refined them into even better accents. The grill to me is more refined and the tail lights added more than a basic one wide light reminiscent of earlier years of cars. Do I need to say anything about the 1971 Cuda? The Dart however maintained the same grill and tail lights from 1970 into 1971. The Demon again was essentially the same as the previous year, but both were awesome and the 72 grill for Dodge A-bodies was a change for the worse in most eyes. The Plymouth Scamp came along and stepped into the A-body race beside the Dodge Dart in 71. It is a tough call between those two grills. But if you toss the Duster Cheese-grater in there, there isn't really a question!
I'm a 71 lover in almost every model of the two. They were near the end of the desired cars, but they left a massive impression before they bowed down to the future.
Of course, I realize that the world holds the 68-70 Charger as the iconic muscle car, but that is simply all in the eye of the beer-holder!