Worm from Cryptopsy, especially on their live dvd, he has a REEEEEE type growl and I can't understand a damn word he growls. That is what makes me mad about Cryptopsy, I love their music, but I wish they had a better growler.
The best singer I have seen live that switches from growls to clean perfectly is Bjorn from Soilwork. I also personally like Howard from Killswitch. The best growlers/sceamers I have seen live have been Peter Tatgren from Hypocrisy and Akerfeldt. The most intense screaming I have seen live came from Devin Townsend with Strapping Young Lad, they just blew me away and their intensity on stage cannot be topped, plus Hoglan is just a freakin beast. Angela also blew me away and so did Chad Gray from Mudvayne, I think he is a very underrated vocalist, but I bet a lot of people on here don't care for Mudvayne, but I personally love their style and they blew me away live.