It is sad that Djokovic and Federer have only 1 French Open title combined to show for their quite solid clay performances. Both of them, especially Fed, were denied a couple of those titles by the almighty King of clay. Remember some big name stating once "In tennis, you have hard court specialists, clay court specialists, grass court specialists... and you have Roger Federer". Also IMO, Novak is not much better on grass than he is on clay despite having 3 Wimbledons and no French Opens.
In the end, I actually think Novak's and Roger's games on clay are better than Nadal's hard court game. It is just that Novak and Roger were not so dominant on their favorite courts (hard court and grass respectively) to deny Nadal winning those Majors. Nadal though was so dominant on clay, having a combined 11-1 H2H vs Fedovic at RG, with that one loss not resulting in Novak winning it this year.
Though Nadal would be the weakest guy for me in this discussion, he has the most titles on least favorite courts to show otherwise. So this is the most contradicting statement I have ever made, but that is tennis.