Of course, most of these comparisons occurred a while ago, when Rafa was very much regarded as a clay-court specialist. Since then, as far as I can see, most have conceded that he is a second Superman, rather than Superman's Kryptonite.
And now we have entered the era of Rafa's rule over the tennis kingdom. And it brings about the question of: Who will be Nadal's Kryptonite?
I think many would argue that for the moment, that man is Roger Federer.
But I disagree. Roger Federer is still Superman, and the Original Superman at that. So how can he be Kryptonite as well?
Plus, you need only look at his track record against Rafa to see that he just isn't capable of it. This is not a slight at Roger, but a compliment to Rafa and his current form.
He will not knock Rafa off his throne; of that I am certain. Rafa knows how to win against Roger.
Many a Brit will argue that Andy Murray is well on his way to becoming the bane of Rafa's career.
He's beaten him, what? Twice in seven career meetings? (And notice I say career meetings.)
Take the US Open semifinal. Rafa was coming in at the end of an incredible season. His body had taken a battering, and he was physically exhausted. He has never played particularly strongly at Flushing Meadows.
In the third set, Rafa gets that break. Momentum starts to swing his way. Cue match delay. It just wasn't meant to be.
Then take the Rotterdam final. Rafa was injured, badly. That's all I will say.
I say these things not to take away the merit of Andy Murray's wins over Nadal, all credit to him for playing well. But you cannot ignore these factors which, in my opinion, had a large impact on the matches.
Of course, Andy Murray has beaten Nadal when both were rested, fit and healthy. At the Abu Dhabi exhibition.
They both played well; Andy Murray just played better. No excuses. But that one win is not enough to gain him Kryptonite status in my eyes.
And then there is Novak Djokovic. He is capable of challenging Nadal, as are Murray and Federer. But far more often than not, Nadal will come out on top in these meetings. It's the way it is, and the way it shall be for some time yet.
I read an article once (I can't remember where, or I would give credit to the author). It compared Rafael Nadal's current form to the moment in Troy when Achilles turns to the army of Thessaly and yells "Is there no-one else? IS THERE NO-ONE ELSE?" And in my eyes it is an apt analogy.
Because there simply is no-one else. Nobody who can truly make a challenge to Rafael Nadal's throne.
So who is his Kryptonite? Well, it's easy.
He just doesn't have one.