We know why Roxas and Namine are special and I can tell you why Ansem was special...
Ansem isn't a heartless... He's simply a heart, like Kairi was...Unlike Kairi, hhis heart was not pure with light...
Ansem needed a host in order to fight and reveal his true self... Other heartless do not need hosts... Ansem was able to not only live within Riku as a second heart, he was also able to remove Riku's heart and use Riku's body as his own...
One reason why he is a heart, could be that as Xehanort, his heart was pure... Not of light, but of darkness...
Another could be that Xehanort was just so powerful, his heart could not be overpowered by darkness... Perhaps Xehanort's own darkness completely outmatched the heartless' darkness...
Xehanort is a heart... That's why he is special... He himself said that he become a being of only the heart, but he was not a heartless...