NXT because it is a completely new code base. Improvements are faster transactions, name based accounts, in wallet asset exchange, in wallet marketplace, supernet integration, healthy community.
As much as we have seen innovations, most are entirely based upon the Bitcoin code base, or close derivatives, while I do not mean to denounce all altcoins here, there are others. FWIW I'm not a NXT hodler, but I have used the software, it's fast, easy and the integration of marketplace and other community building features makes it more than just a coin. Also excited about the developments of Supernet.