Novak started dominating Nadal truly only last year. Before 2011, Nadal was definitely his hardest opponent off hard courts.
In 2011 his entire game, physical and mental strength came together and he figured out how to outhit and outlast Nadal. Agaisnt this Djokovic, Nadal has nowhere to go. Agaist this Djokovic, Federer has the best chance because he is more naturally aggressive. That isnt the case against just Djokovic, its against more or less everyone. Federer at his best is still better than anyone else on a tennis court. But thats all really. That doesnt make Federer his "real rival". Federer wont be there always, not at 31. Even in a single season, Federer no longer makes the final stages of all tournament like he once did. Thus, against peak Djokovic, he doesnt qualify as the "real rival". But no doubt, Federer proved the guy who can hurt Djokovic most likely in 2011.
Novak does seem to "hate" losing to Fed more, but that has more to do with the history between them. Roger hasnt taken kindly to Novak's on court antics and retirements in the past and often had harsh words (even calling him a 'Drama Queen' once
). But basically, they seem to get along just alright really. Fed-Nadal off court bromance is hardly the prototype.
Federer may be GOAT, but Nole fans shouldnt hype him up as Novak's "real rival". It doesnt say much about your domination if your real rival is a guy who has played over 1000 pro matches alone and is on the wrong side of his 30s...