Mainly Death and Black metal since most thrashers don't scream, the the vocalist for Kreator and Tom Araya are pretty sick.

I'm gonna say

Death Metal:
Chuck Schuldiner(Death)-His progression was awesome, he had the some what low scream/high growl at first and then those high pitched ones for Symbolioc and TSOP.

Kelly Schaefer(Atheist)-Probably the best screaming vocalist, you might not even like Atheist but he's amazing.And on their last two albums he would scream very fast and sometimes would actually talk which just made the songs awesome.Even if you don't like them check out the first verse of their song "Air" and the first verse of "The Formative Years"

Frank Mullen (Suffocation)-In the 90s he was really guttural and now he's guttural and clear.In general he's just amazing.

Mikael Akerfeldt-He's clear and still really low with Bloodbath.And unlike most DM bands who use clean vocals HE CAN SING.Beautifully too.

Black Metal:
Varg Vikernes (Burzum)-He has such evil vocals.Really high but not high on Belus, still good though.On some songs he's even a little low like on the song Burzum/Dunkelheit.

Attila Csihar-He's amazing.He croons,snarls,shrieks,growls.You don't know what the fuck this guy is going to do EVER.He's always changing.With someone like Corpsegrinder you know he'll scream and growl.Attila is different every song.

Abbath(Immortal)-He sort of made what could be considered the basis for black metal vocals, they're sinister sounding and easy to do.But sometimes he can be emotional too.Like on the Sons of Northern Darkness album he just slays in the longer songs.

Satyr(Satyricon)-The first 3 albums he's amazing.His vocals remind me of a mix between Attila's clean vocals and Abbath's black metal vocals with a little Satyr flavor thrown in.He's just awesome in general.

I really like Silenoz on the first Dimmu album his vox were pretty sick.

I don't know about extreme metal, but there's always Nils Frykdahl of Sleepytime Gorilla Museum who has an almighty baritone snarl.

Eh, it's a hard question really.

I like Mikael Åkerfeldt from Opeth and have done for several years, but truth be told I rather prefer his singing to his death metal parts. Similar tale with Paul Kuhr of Novembers Doom.

Burzum are one of my two or three very favourite groups at this point in time and yet I can't say I really rate Varg's vocals as anything spectacular.

Attila Csihar is a very good shout, I'd definitely consider him up there, he always sounds interesting.

I dunno, I'd also consider Niklas Kvarforth from Shining (the Swedish black metal one) which is just emotive. He's great on the Shining records & great on the album he did with Bethlehem (A Sacrificial Offering to the Kingdom of Heaven in a Cracked Dog's Ear)