I’d go with 2011 personally. Even though his 2015 season was probably even more impressive statistically, I feel like Djokovic’s 2011 run was the pinnacle of tennis. Novak dominated the year with only 2 losses until September, although his competition was prime Nadal, prime Murray and near-prime Federer amongst others. Djokovic even dominated Nadal on clay which was completely unheard of at the time. Best individual season I’ve ever seen since I started watching tennis about 18 years ago.
Voted for 2015 because it was the best season as a whole.
While 2011 was slightly better up to USO, he fell apart pretty hard afterwards, and he swept everything post-USO in 2015.
In 2015, I can say the Djokovic was best player on every surface, hardcourt, clay, grass and indoor. In 2011, he wasn’t the best on indoor, that would be Federer. And Nadal won RG and Monte Carlo in the clay season.
I also hold ATP finals in very high regard. Not only is it the 5th biggest tournament of the year, it is basically a trial by fire and the winner is truly the “best of the best”. So for me, 2023 is close to 2011 even though he was far inferior in masters performance.