Level of play: Nole 2011. A dominant 3 Slam season in the most competitive years of tennis(2011-13).
Achievements: Nole 2015. 3 Slam + 6 Masters + WTF. Federer 2006 is close(2 less Masters, but 2 more overall titles).
So I'll go with these 3.
We can obviously take Laver 1969. The CYGS. But honestly, even if Nole would had beat Medvedev in USO final, I was not going to take 2021> 2015 in terms of achievement.
It's hard to compare 1969 against a recent season. It technically was in Open Era, but a lot of dynamics were like pre-open era in late 60s.
2 less masters but just 1 more title:
2015 - 11 titles: 3 slams, WTF, 6 masters, 1 MM
2006 - 12 titles: 3 slams, WTF, 4 masters, 4 MM