Fed seems the toughest by a good margin.
I used slams and not failing to get far in them as one criteria.
Fed made 18/19 finals in slams and didn't let down mentally to make them for so long.
Nadal would not keep his intensity, in the later parts of the year he would not stay tough.
Djokovic is about the same as Nadal, has trouble keeping himself at top level through the whole year, even in 2011.