What genre would describe the music of Nusrat Fateh Ali Khan (the late Pakistani superstar singer) and who are the current top singers who authentically embody that same style?
As @wcullen says in the comments, Nusrat Fateh Ali Khan performs qawwalis in the Sufi tradition. The Encyclopedia Britannica has a reasonable article on qawwalis, although the description of the style of music is given from the perspective of Western classical music.
Nusrat's nephew Rahat Fateh Ali Khan continues the tradition of performing qawwalis, in addition to singing ghazals and performing as a playback singer in several movie industries. The Sabri brothers are another troupe who are of the highest caliber. You could take a look at the music of the Wadali brothers and the Bandanwazi Qawwal troupe as well. It seems they too come from very strong Sufi qawwali traditions. Also, Wikipedia has a partial list of qawwali performers, so you could search there for similar contemporary artists.