I think he played worse in 2015 because he didn't adjust his game to Wawrinka at all. In 2014 he was just playing Nadal, and last year Nadal was still playing well enough to win. It also looks like the very hot weather in the Final affected him last year, which can be viewed as an excuse.
He has never played as well as in 2013 though. That match could have gone either way, but Rafa just refused to let Nole take the trophy in that 5th set, despite being broken early in the set. I also think Nadal hit as many winners in that set alone as 1/3 the winners Wawrinka hit in all match yesterday. Crazy stuff.