Can't speak for later Enslaved, but I'm a big fan of Vikingligr Veldi and there's nothing special about his vocals imo. Weird choice for the best of the genre.
Nocturno Culto (especially on A Blaze...) gets my vote for the best black metal vocalist. Absolutely fucking wretched, disgusting and depraved. His vocals on Transilvanian Hunger are also worth noting, incredibly haunting. And of course the sheer balls of his vocals on Panzerfaust.
The vocals in that Trivium song were painful in a bad way and it sounds like he's straining incredibly hard.