My favourite is Blizzard Beasts, as for best I couldn't be sure. I like all of them, though Battles in the North, Damned in Black and Sons of Northern Darkness can bore me if I listen to them too much.
I think "Sons Of Northern Darkness" is BY FAR their weakest effort. It's just too.. boring. The only really good track on "Sons.." is, IMO, the title track. "Damned In Black" is often criticized by fans of extreme metal, but I think it's a very solid release.