Why is Kurapika so powerful? I know he made ​​a contract with his own soul or something like that.  But if this Nen is so powerful, how come he is the only who used it in the series?  

I also watched the previous version(1999), and there was no one who did this type of contract besides him.
Kurapika's scarlet eyes allow him to shift from becoming a Conjurer to a Specialist because of their unique nature. This ability, Emperor Time, allows him to utilize all the types of Nen to 100% efficiency. 

Kurapika has a Nen blade chain wrapped around his heart. Kurapika imposed a limitation onto this chain, it can only be used to deal with members of the Phantom Troupe. If this condition is violated, Kurapika will die. This condition (a Limitation, a Vow with a punishment) was needed in order to increase the strength of the ability.