...given the circumstances, match level, opponent, stage, etc, all things combined?
I think it's either the FO 2011 semifinal against Djokovic, the Wimbledon 2012 semifinal against Djokovic, or the AO 2017 final vs Goatdal
FO 2011 SF: his weakest surface, tough year, opponent on the historical streak, high-level match
W 2012 SF: KO match for the number one spot, back problems during the tournament, more than 2 years without winning Slams
AO 2017 F finally won a Slam, beating his kryptonite in the final after a long break. However, it has to be said that Nadal back then wasn't close to King from 2011-12
Other contenders:
I think it's either the FO 2011 semifinal against Djokovic, the Wimbledon 2012 semifinal against Djokovic, or the AO 2017 final vs Goatdal
FO 2011 SF: his weakest surface, tough year, opponent on the historical streak, high-level match
W 2012 SF: KO match for the number one spot, back problems during the tournament, more than 2 years without winning Slams
AO 2017 F finally won a Slam, beating his kryptonite in the final after a long break. However, it has to be said that Nadal back then wasn't close to King from 2011-12
Other contenders:
- Wimbledon 2001 vs Sampras: Youngerer & Chokerer against the King of Wimbledon. Not the best year of Petros, but he was still the defending champion and 1st seed. The modern equivalent would be Sinner defeating King at AO2024.
- Hamburg 2007 F vs Nadal: ended Goatdal's streak on clay, first time beat Rafa on the surface, served a bagel in the decider
- Wimbledon 2007 vs Nadal: defended his territory from good Nadal, although Rafa was still pretty young
- US Open 2008 SF vs King: defeated a good King, avenged his AO loss, proved he still has it.
- FO 2009 SF vs Del Potro: defeated peaking opponent on the way to win 1st RG, won the match despite being outplayed
- 2010 WTF F vs Nadal: defended WTF from Indoors mug who had the best year of his career
- Cincinnati 2012 SF vs King: obliterated peak King, served a bagel to him
- Shanghai 2014 SF vs King: dominated peak King in two sets
- Wimbledon 2015 SF vs Mugray: straight set win against peak Mugray
- Shanghai 2017 F vs Nadal: completing 4-0 season sweep, winning all 6 sets in their MS matches
- Wimbledon 2019 SF vs Nadal: another win against good Nadal in style
- WTF 2019 RR vs King: straight setting King at 38 years, eliminating him from WTF, and ending King's hopes for YE#1