(For the record, I still think the Big 3 will end up in the current order, ie Federer > Nadal > Djokovic, but things are not as straightforward as they look, and certinaly not as clear cut as 20 > 17 > 16, imho. Sure, some younger people bought into the Sampras narrative that "slams are everything", but let's be real, no-one in their right mind ever ranked Sampras above Laver. Heck, some even rank him below Borg. The ATP is currently pushing that narrative too because it's the one that makes the race look interesting, but they will change it as soon as they find another one that they feel is more exciting.)
1) Slams. No surprise there, everyone knows the totals by heart, with Nadal 3 slams behind Federer and 4 slams ahead of Djokovic. At face value, 4 > 3, but that's before the following kicks into motion.
2) Majors. The only real way of counting achievements if you also want to consider the earlier eras of tennis. Problem is, they're not equivalent and they've changed over the years, so some leeway is obviously needed. Still, when counting majors:
- Nadal doesn't add anything to his tally (or adds 1 because of OG; and although Olympics was never important in tennis, it's starting to become so, so I would be tempted to give it to him, for a major total of 18).
- Djokovic gains a whopping 5 (YEC), for a total of 18 (ties Nadal in that respect).
(Note: this doesn't mean that they're equal, as 1 major isn't worth 1 slam, obviously. Still, they have to count for something.)
3) Dominance. Djokovic is much stronger than Nadal in that respect, with two clear (although not very long) periods of dominance, but Nadal is catching up thanks to the perfect storm that was 2017.
4) Surface/condition analysis. (Yeah, I know this could be seen as basically another take on the "Rafa's resume is clay-skewed" argument.) Of the four surfaces/playing conditions, at the moment, Djokovic has got two at GOAT level and two at ATG level, while Nadal has one at GOAT+ level and two at ATG level, ie:
Nadal:
- clay: GOAT+ (ie best ever)
- grass: ATG
- HC: ATG+ (close to GOAT)
- indoor: irrelevant
Djokovic:
- clay: ATG
- grass: ATG+ (close to GOAT)
- HC: GOAT
- indoor: GOAT
(By way of comparison, here is Federer's tally and why his resume is so impressive: )
Federer:
- clay: ATG (would have been GOAT without Nadal)
- grass: GOAT+ (best ever)
- HC: GOAT+ (best ever)
- indoor: GOAT+ (best ever--although this may be disputed as there are much less indoor tournaments than there were before; still, at least GOAT)
5) Other. The four slams in a row by Djokovic (2015-2016), although not a calendar slam, are obviously worth something, too.
So, to wrap it up, if we count two YEC's as one slam (roughly) and add another one for the fact that Djokovic once won four in a row, even without factoring in things like dominance or mastery of the various surfaces and playing conditions, we get to something like 23 > 17 > 16, which seems closer to the current tennis landscape and the respective careers of the Big 3, imho.