Okay in summary
1/ Best season is not solely determined by win loss record. It is just one factor. Djokovic's win-loss record is about the 5th best in the open era behind McEnroe 1984, Connors 1974, Federer 2005, Federer 2006
2/ Best season is mostly determined by achievement ie top titles won.
Djokovic's season certainly rates - I would place it around 3rd best in Open era - behind Laver's 1969 and Federer's 2006 (Federer won one less Masters 1000 but he won the WTF which is more important - 4 Masters 1000 + WTF > 5 Masters 1000; plus Federer did better in the Slams and won more titles overall).
Hence, Djokovic's season probably rates around the 4th or 5th best in the Open era. Well done - fantastic year.