e = mc^2 says a lot more about the universe than e = ((pc)^2 + (mc^2)^2)^.5
Before someone rants about how setting p=0 gives you the form blah blah blah... stop. I've been there before. That's still not the equation that made him famous. It is the simplified version which speaks to the direct relationship between energy and mass. It's elegant and simple enough for the wide of public culture to consume.