I would say the Top 3 contenders for Nadal's best year are:
2008 - 8 titles - 82-11 (88%)
Big titles: French Open, Wimbledon, Olympics, Davis Cup, Monte Carlo, Madrid, Canadian Open
2010 - 7 titles - 71-10 (88%)
Big titles: French Open, Wimbledon, US Open, Monte Carlo, Madrid, Italian Open
2013 - 10 titles - 75-7 (91%)
Big titles: French Open, US Open, Indian Wells, Madrid, Italian Open, Canadian Open, Cincinnati
He does have many other great years including:
2005 that had his most titles won in a season (11)
2018 that had his his highest winning match percentage (92%)
2017 and 2019 where he also won 2 slams in each.
And who knows, maybe 2022 will be his best year yet.
2008 - 8 titles - 82-11 (88%)
Big titles: French Open, Wimbledon, Olympics, Davis Cup, Monte Carlo, Madrid, Canadian Open
2010 - 7 titles - 71-10 (88%)
Big titles: French Open, Wimbledon, US Open, Monte Carlo, Madrid, Italian Open
2013 - 10 titles - 75-7 (91%)
Big titles: French Open, US Open, Indian Wells, Madrid, Italian Open, Canadian Open, Cincinnati
He does have many other great years including:
2005 that had his most titles won in a season (11)
2018 that had his his highest winning match percentage (92%)
2017 and 2019 where he also won 2 slams in each.
And who knows, maybe 2022 will be his best year yet.