While I agree to a degree I would content that Nadal-Djokovic rivalry is the most enthralling in tennis today. Their grinding battle of attrition is actually pretty exciting. Considering Federer is now beyond his prime I really don't get excited by any of his so called next gen rivals. Despite the successful aberration of an year he has been having, he won't be able to sustain it next year -- I feel it is his last hurrah. It's almost a given that he will end of with a negative H-H against Nadal, Djokovic, Murray et al. if he persists in playing for next few years further into his decline.
Nadal/Djokovic was exciting the first couple of times last year, if only because it was great to finally see someone who could beat Nadal at his own game. But AO 2012, far from being an epic that it's touted to be, was a boring, one-dimensional slugfest that quite aptly represents the rivalry. A tennis match does not have to be 6 hours long to be epic; it does not have to be 6 hours long to be a good match. I personally found it really boring and I really wasn't a fan of the tennis on display.
Federer/Djokovic, on the other hand, benefits from the unpredictability mentioned by rickcastle and also the wider variety of shots that both players are able to bring to the court, though these days that tends to be Roger more than Nole. Their matches at the French and US Open last year were some of the best of the year. Conversely, some of the Nadal/Djokovic finals like Wimbledon and the US Open, I didn't find to be of particularly high quality.
As for Federer/Nadal...the less said about it, the better.