Federer, Nadal, and Djokovic are all amazing players who are doing things that, judging by the historical record, they really had no right to do. As already discussed in this thread, Murray not quite so much, but I was really glad when he got his first slam, when he became a multiple slam winner, and when he finally reached No 1. All incredibly hard-fought achievements. And for a brief time Wawrinka turned the talk to a Big Five with three slams in three years.
Anyway, my career ranking would be:
1. Federer
2. Djokovic
3. Nadal
4. Murray
5. Wawrinka
My personal ranking would be much the same. The ones I particularly like to watch are Federer and Djokovic, but also Wawrinka when his backhand is going well. Federer's the only one I've seen play live, though honestly you can see a lot better on TV than from one of the back rows in Rod Laver...
To me, one of the amazing things is the longevity. I remember when Federer's 2012 Wimbledon win was massive, the thing that number crunchers said was probably going to help him just keep an edge over Nadal, and a bit later when Wawrinka was rare in winning multiple slams over 30. For example, back in 2014 Five Thirty Eight ran an article
Happy Birthday Nadal! You’re Probably Too Old to Pass Federer (and that was even if Federer's count stayed at 17). And now somehow it's become
normal to win multiple slams over 30.
At the start of 2017, Djokovic seemed to have lost his edge a little, while Federer and Nadal were both on the comeback trail. A Fed-al final was talked about as the fairy-tale finish, but it didn't seem that likely. Not only did it come about, but it was an awesome final. And then Federer and Nadal split the next six slams between them. Federer got to No 1 (briefly) at 36. And now Djokovic has won the last three slams, so the relative slam count is exactly what it was at the start of 2017. That really wasn't how it was meant to work (and yes, does raise a concern about weakness in the generation below).
So it's true that Federer won lots of his slams when he was in his prime and there were fewer contenders (though you can only play the person on the other side of the net). But the flip side is that in recent years he's been competing with the rest of the Big Four when they were in their prime and he was five years older. And in the Slam-less years between 2013 and 2016 he was still frequently making slam semis and finals, spent a lot of the time ranked No 2, and remained competitive while his early competitors and peers retired.
So to me assessing it right now it's not just that Federer has 20 slams and has been No 1 for most weeks, but his general longevity and consistency. The number of consecutive slam quarter-finals he made. The fact that he won slams and was No 1 at 36. Those are really rare achievements.
I give Djokovic a slight edge over Nadal because his performance is a little more all-surface (and winning all the masters certainly helps). Even more so if he makes a double career slam at the French Open, which is a possibility (though, to be clear, Nadal has had plenty of success on grass and hard-courts, and both Federer and Djokovic have had success on clay - it is hard to separate the three). I also think it a little more likely that Djokovic will rival Federer for longevity than that Nadal will, and if he does it's much more likely that he will shoot to No 1 in my book.