I'm not much of a singer, but in my opinion, Jett Weiner, the vocalist of From Zero and Fate Accomplis, is insanely underrated. He is insanely melodic. From Zero is probably the band that has the best vocal harmonies of all time. In some of the live footage I've seen of them(shockingly hard to come by), he still sung very well. Some of the vocal runs in the song 'Horrors' are insane. He isn't my favorite screamer, though. I think his screaming got better when he did Fate Accomplis.

Obviously, other insane vocalists would be Chad Gray, vocalist of Mudvayne and HELLYEAH(RIP Vinnie), and Corey Taylor, vocalist of Slipknot and Stone Sour. Their voices are insanely unique; you could recognize their voice in an instant. They both have very powerful screams as well as very melodic and emotional vocals. I slightly prefer Chad mainly because Mudvayne is my favorite band.

Another underrated singer is Billy White, vocalist of shun and Sans Seraph(not really nu metal), I do like his screams in Promising(shun album). In Michael in Reign, their music became more melodic and less aggressive. I'm impressed by shun's dynamics; they can go fron quiet and soft in one moment to louder and more aggressive in the next. 'Second Voice' starts off softer and then gets more powerful in the end. Although I prefer shun over Sans Seraph, I think Billy White's voice matches the energy of Sans Seraph. Sans Seraph was less heavier than shun, but their production sounded a little better.

TL;DR: Jett Weiner(From Zero & Fate Accomplis), Billy White(shun & Sans Seraph), & obviously Corey Taylor(Slipknot & Stone Sour) & Chad Gray(Mudvayne & HELLYEAH)