I think you could make a case for it being close when Roger or Murray wins the WTF, but if Djokovic wins it's quite clear. I think rankings in the end are very fair. If Murray wins it he has won OG, WTF AND a slam, plus a final. That's the most big tournaments. If Roger wins, he's won a Slam, the WTF, a silver medal, 3 MS titles, and maybe 3 500's as well. That's a strong case too, but Roger has somewhat underperformed in Slams compared to the others.
It's quite close, and the rankings will reflect that. I think rankings ultimately are very fair in men's tennis.