Now I'm a big Federer fan, but I have to say the crowning achievement has to be the Calendar Slam . Correct me if I'm wrong, but isn't Rod Laver the only one to have won all the grand slams in the year successively? I know this isn't a 'Is Roger the GOAT?' thread but since we're on the topic of records/achievements etc I'm just saying this has to be the toughest task in the modern era and Roger hasn't been able to do it. Speaking for myself, Roger will become the GOAT only if he makes a Calendar Slam. I know Rod Laver had only 10-11 slam titles but the magnitude of completing the calendar slam is such that it overshadows everything else. (olympics, career slam etc, and yes even 16 grand slam titles)
But during Rod Laver's time, 3 slams were played on grass. So, for me, it's not much of a big deal.
I have more respect for Agassi, Federer and Nadal who all won the Career Slams in different surfaces.