A Calendar Grand Slam is about winning all 4 slams in a year. Winning 3 slams in a year in any order is not important. What's important is how he/she fares against the field in 28 matches. At the end of the year, the performance and result out of the 4 slams dictate who was a superior player that year.
Federer 2006 outclass Djokovic 2015 in all 4 slams combine:
1) Federer was forced to a 5 set 1 time, Djokovic 3(edge Federer)
2) Federer handed out 10 bagels, Djokovic 4(edge Federer)
3) Federer won 15 matches in straight set, Djokovic 19(edge Nole)
4) Both player's loss were at the FO final in 4 sets(edge even)
This comparison is closer to the one with Federer 2006 vs. 2021 in which Federer was superior over Djokovic by a landslide base-on the stats.
While both had a phenomenal performance and result across all 4 slams, I give Federer a slight edge to Federer 2006 over Nole 2015, added to the fact that he face clay god in the FO final while Djokovic had Wawrinka.
Federer 2006 outclass Djokovic 2015 in all 4 slams combine:
1) Federer was forced to a 5 set 1 time, Djokovic 3(edge Federer)
2) Federer handed out 10 bagels, Djokovic 4(edge Federer)
3) Federer won 15 matches in straight set, Djokovic 19(edge Nole)
4) Both player's loss were at the FO final in 4 sets(edge even)
This comparison is closer to the one with Federer 2006 vs. 2021 in which Federer was superior over Djokovic by a landslide base-on the stats.
While both had a phenomenal performance and result across all 4 slams, I give Federer a slight edge to Federer 2006 over Nole 2015, added to the fact that he face clay god in the FO final while Djokovic had Wawrinka.