Djokovic-Nadal is a great rivalry which really took off in 2011. 17 of the last 19 matches in finals, with several GS title fights and the de facto final at RG 2013. For me, it's one of the Top 5 of the Open Era easily - I'd rank it either fourth or fifth alongside Mac-Lendl and behind Laver-Rosewall, Borg-McEnroe and Federer-Nadal in some order.
It's possible that Nadal-Djokovic can climb higher if they contest a few more classic finals in the next few years. It's already ahead of Connors-Borg, Sampras-Agassi, and other great, era-defining rivalries of the Open Era.
In any event, for me the quality of tennis matters a lot, but so do the stakes, and one clue is to look to see if the players kept meeting in big finals over and over again across individual seasons. Federer-Djokovic, for instance, is a great rivalry for the diehards (wonderful tennis and two alpha males butting heads), but has never been era-defining. Just not enough GS finals or streak of meeting in finals across the tour for my taste.
Anyway, for the sake of discussion, here are my Open Era candidates for top rivalry, in my opinion (obviously some of these can be ruled out - I'm just trying to capture the matchups that most often generated finals over a concentrated period):
Nadal/Djokovic from 2011-14 (17 of 19 matches in finals; 6 GS title fights and de facto final at '13 RG SF)
Fed/Nadal from 2006-10 (17 of 19 matches in finals; 7 GS title fights)
Fed/Roddick from 2004-06 (6 of 7 meetings in finals; 3 GS title fights)
Sampras/Agassi in 1995, and 1999-2002 (11 out of 16 meetings in finals; 4 GS title fights and arguable de facto final at 2000 AO SF)
Becker/Edberg from 1988-91 (13 of 17 meetings in finals; 3 straight Wimbledon finals)
Lendl/Wilander from 1985-88 (8 of 10 meetings in finals; 4 GS title fights)
Lendl/Becker from 1986-91 (11 of 15 meetings in finals; 3 GS title fights)
Lendl/Mac from 1982-85 (18 of 21 meetings in finals; 3 GS title fights; multiple Dallas/YEC finals back when they were bigger than or equal to AO)
Mac/Connors from 1980-83 (10 of 14 meetings in finals; 1 GS title fight; multiple Dallas/Philly title fights back when they were bigger than AO)
Borg/Mac from 1979-81 (9 of 13 meetings in finals; 4 GS title fights and a '79 Dallas final meeting in one of the most competitive fields Dallas ever produced)
Connors/Borg from 1976-79(12 of 15 meetings in finals; 4 GS title fights and several YEC/Philly/Pepsi GS finals)
Nasty/Smith from '71 through '73 (6 of 7 meetings in finals; a classic Wimbledon final and a few finals at the YEC and big indoor events of the day)
Laver/Rosewall from '68 through '72 (12 of 17 meetings in finals; 2 GS title fights, 2 Dallas finals - including their '72 classic, and big finals in Sydney, Philly etc.)