Federer - Djokovic is the most entertaining rivalry in history of tennis, for me there is no doubt. Because they are more similar as players than most people realize - most people presume that Federer is a constantly aggressive, offensive machine/maestro and Djokovic a defensive, counterpunching wall.
But, they are actually similar in some ways. Both of them try to stay close to the baseline, hit the ball on the rise with incredible timing(both are very talented in that regard). With their court positions and ability to take time away, it often felt like watching a table tennis(ping pong) match with two incredibly skilled, precise ball strikers. Federer's serve(the best spot server of all time, probably with Pete Sampras) against Djokovic's return was the most interesting part of it, at least for me. Also, Federer taking pace off the ball with the slice, inviting Djokovic to be more offensive, dragging him sometimes with the shorter slice to a no man's land where Novak doesn't feel the most comfortable etc. Matchup full of tactical nuances.
In general, they both liked lower bouncing, indoor(if possible), faster surfaces(especially Federer, Djokovic is a bit more adaptable/complete - surface independent). For those who don't understand why Djokovic prefers faster instead of slower conditions - his offense works way better on slicker, faster, low bouncing courts where his pace generation "averageness" is not exposed at all and his defense(because of incredible pace absorption, compactness of strokes and speed) is relatively uncompromised, even on fastest of courts - his effortless sliding/movement on grass is another evidence to that.
Rivalries with Nadal that they had were mostly surface dependent, and makes it a little bit boring/predictable. For example, I knew that Nadal is going to trash Djokovic in RG 2020 final where conditions were so, so slow and heavy(held in October, colder time - autumn) that Djokovic can't hit through them and Nadal obviously could because he's the greatest pace generator of all time, built like a bodybuilder. Also, I knew that Nadal had no chance against Djokovic in AO 2019 final on a faster, low bouncing hard court where Djokovic's technical brilliance, compactness, precision would outmatch Nadal's physicality, muscle(strength, pace generation), constantly rushing him and taking time away. Novak was mostly dominating that rivalry post 2011, even seriously challenging Nadal on clay(beating him twice at RG).
On the other hand, Nadal was mostly dominating the Fedal rivalry, by relentlessly exploiting Roger's one hander with that lefty, heavy topspin forehand over and over and over again. Using one same pattern to win most of the matches. That's not very interesting from a tactical standpoint, we all must admit. Federer figured him out(outside of clay) in 2017 when he learned to step in and take that backhand on the rise(thing Djokovic does, another example was Davydenko) instead of waiting to bounce at or above his shoulders(thus bothering/compromising Federer's contact point) and slicing it, but it was too late.