Nadal is amazing on hard courts when he's at his top level, but when he plays average, he's much more vulnerable than on the others surface. Djokovic is not better on grass, probably worse despite his Wimbledon title.
Federer on clay = Nadal on hard (the field is way tougher on hard courts than on clay) > Djokovic on grass > Murray on clay
Federer on clay > Nadal on hard.
Remember, without Nadal (aka the best claycourter ever), Federer would have won at least 5 RGs. And Federer has defended clay title.