I cannot really make a ranking, but I try to say something about anyone here.
Djokovic:
- Yes, he has proven it for the third time against Federer after 2 match points down. That is unprecedented in tennis history I think. I also remember many other tough situations he handled on his way to Slam titles.
- The only restriction is when he refuses to play serious tennis and is only interested in other things (discussions with referees or the crowd etc.) Then he is quite absent from the game and cannot activate his mental strength on the court (like in the Thiem match at RG, and even there he almost saved everything in the end).
Sampras:
- Despite being a fan and having watched many of his matches in my childhood and youth I must admit that I don’t remember that many close AND meaningful matches. He is way behind the big 3 in terms of being included in those matches, so it’s hard to judge. Many of his 5-setters were just early round struggles on clay or later in his career.
- His greatest remembered moments came when he was psychologically (crying at AO 1995) or physically (vomiting at US Open 1996) down and still somehow prevailed.
- He was generally great in quickly saving 0-30 or break point situations with service winners, but he could be shaky with the volley if it mattered most (for example against Federer 2001).
- He lacked the mentality of choosing the right shot selection on a court where attacking immediately wasn’t right. And it got worse when the match became closer with desperate chip-and-charge on clay etc.
- He would have never saved a decisive RETURN game in Djokovic style.
- All in all as I said, there are too few situations to really judge. It’s like Borg a bit mythical ("Sampras would just serve 5 aces when being 0-40 down to save a match, and if needed he even does it with the 2nd serve"). But where are these moments (apart from the one big match point save against Corretja 1996)?
Nadal:
- Absolutely great mental strength when the conditions generally favour him and he just has to come back into top form (like the 5th set against Djokovic at RG 2013).
- Significantly lesser when there is more needed than just bring his usual patterns back into play. If he tries to do something different, he tends to make crucial double faults or other mistakes. It was not only the strength of his opponents that he lost all those close grass matches in the last years.
Borg:
- It was before my time, but I have watched his Slam finals and a few other big matches on Youtube etc. Due to some occasions, especially the 1980 US Open final, I see his mental toughness as overrated (not so sure by which extent though).
- If we look at that match, he failed to serve out a set 3 times and hit crucial double faults in the 5th set. At a time he totally lost it and conceded 6 service games in a row! And that happened while trying to win the big title that eluded him. His mythical “ice-cold” mental toughness should have looked different.
Federer:
- Unfortunately he is shaky in tough situations, and visibly so. Some people will say ("but he has 20 Slams"). Yes, but he would have many more with ATG mental strength against big opponents. If mentality were just EVEN between him and Djokodal, he would have at least 25 Slams.
- Another kind of mentality is great though: He seems to have no doubt BETWEEN matches or before the start (or even after a loss he seems to always think he’ll beat that opponent next time). This is what some people call "arrogance", but I think with his abilities that is justified and it helped him a lot.
- He is quite good in beating lesser opponents after 0-2, IF none of the next 3 sets gets really close. Then he seems to just think: "Okay, I should beat that guy 3-0 anyway, so why not start now?" He also can turn it around from match point down against bigger chokers like Cilic or Monfils.
Laver:
- I cannot really say anything here, because there are very few of his matches watchable. But completing the CYGS 2 times (no matter how much 1962 really meant in the history of the game) and overcoming critical situations on the way definitely requires mental toughness.