FF7 said: Emperor, Watain,and Wolves in the Throne Room are my personal favorites.
I second all of these bands, and would add:
(old) Samael (new Samael is good, but it's industrial)
Arcturus (newer stuff is a bit more avant-garde)
Sigh (except for Gallows Gallery)
If you want some viking type stuff, try Borknagar, Enslaved, and Bathory
"Blood on Ice" is a mandatory listen. I'm not trying to undermine other releases (besides "Octagon") but it's Bathory's best in my opinion. Whether you'll like it the most or not, it's amazing nevertheless.
For more viking/folk, try Isengard and Storm. They're not exactly the "most evil" but classic folk metal at it's best with black metal legends in the bands. Kerbenok, Crystalmoors, Agalloch, Draugnim and Kampfar are great ones, too with a more black metal sound.
Good folk metal, black metal-tinged or not, has most of the melody in the guitars; without having to rely on violins or flutes or whatever. Keyboards are generally fine in this type of music, as long as they are used well and not overpowering.