NAD said:
For anything else though I like a nice varied attack like Mikael from Opeth or Peter from Hypocrisy.
I think Akerfeldt is a great vocalist - regardless of how Opeth have changed over the years, his vocals have always been a selling point for that band. He's a good guitarist too.
In brutal death metal, I like the vocals in bands like Nile. Really guttural noise, as if someones reached down their throats and ripped out their entrails. Having said that, the technicality in the music matches them perfectly (making up for the 'simplicity' in the vox). If you take Mortician, they're ok, but you only need one of their albums to get the idea of what they're about.
Thrash vocals have to really be pronounced - almost matching the riffs that are blasting out, like what Max Cavalera used to do. Another guy that's good at doing that is the lead vocalist for Suicidal Tendencies - really punctuated delivery.
The thing I have most difficulty with is melo-death vocals just because the music is so melodic (and borderline power metal) that the harsh screams don't work as well (especially when delivered over a quiet acoustic guitar or whatever). That's why I sometimes see Melo-Death as 'Traditional Metal gone wrong', heh. (See: Omnium Gatherum, or Johan Livva).
Essentially, harsh vocals should typically match harsh music - OR, put even more simply, vocals should match the emotion in the underlying music. I would NEVER want to hear Iron Maiden songs sung with harsh/grim vocals - it always fails, and that's why I avoid most tribute albums like the plague.
P.S. I don't know anything about hardcore other than I don't like it - but to me it's an outside genre, along the lines of jazz or punk, etc. It's not metal to me whatsoever, even though some blurring tends to occur. Rather than diss it, I just move along which I think is fair enough.