If you think about it, compared to every other tier 1 great of the Open Era, he doesn't really have one. Borg never won the USO, Sampras never won the French and was lacklustre on clay in general, Nadal never won the WTF and has a mediocre record indoors(not to mention one or two other things truth be told) and Federer's is obviously his poor H2H against Nadal. With Djokovic however, and especially now he's completed the Career Grand Slam, it's difficult to really pick one since he also has a winning H2H over all his main rivals and his resume as a whole looks so incredibly well balanced. I suppose some of you might say the Olympic gold medal but I'm not sure that's such a big deal given that it's only played once every four years and tennis as a sport has only been part of the event since the late 80s. I guess if I were really pushed to choose something it would be only winning one slam per season between 2012-14 since they were smack bang in the middle of his prime years but even that might be scraping the barrel considering it's a player's overall achievements that matter the most, not one specific time frame.