It's hard to label 'any shot' as the most legendary of all time. We couldn't even agree on the most legendary match of all time, not alone a single shot.
However, that return winner happened when everything seemed to be over for Djokovic. I'd call that shot maybe one of 'the most dramatic' shots of all time ... I still think that had Fed stayed composed and calm he would've closed that match. Remind you, it was still a match point (40:30) after Novak hit that FH return winner. Even at deuce, Fed was still serving ... but we all know what happened.