I voted for Federer. But Djokovic probably has 4 or 5 really good years ahead of him yet. Hes got 5 years on Federer who in reality is the next best player in terms of still challenging for titles. Nadal apart from Clay has been on a downward spiral for a while and Murray just falls short against the better players.
I think Djokovic is still good for perhaps another half a dozen slams, maybe even more. I'd fully expect Djokovic to surpass Sampras and Nadal in Slam wins, whether he can catch Federer though is open to debate.
PREDICTIONS FOR END OF 2015: (Made Jan 1st 2015)
PS4 - 34M - XB1 - 21m - WII U -12M







