i think i agree. djokovic's 2011 was better than any of the best years from edberg, becker or courier. connors, borg and wilander also had years where they were clearly number one over the field. but i think djokovic 2011 edges them all just a bit because of the output of 3 slams, the overall w-l record and the h2h against his rivals.
the problem with nadal is that he never starts and finishes the year well. even his best year 2010, he retired in australia and fizzled out as usual in the yec. i think overall, djokovic's 2011 slightly surpasses nadal's 2010. agassi 1999 was also great but he still lost more matches than djokovic 2011 and i think sampras still owned him head to head that year.