well i can't say i've heard them all (basically everything between perdition city and blood inside), but i'm torn between bergtatt and kveldssanger as my favorite, but i'd have to say that their strongest is perdition city. just a solid fuckin album.
eh actuallly im gonna vote for kveldssanger, the album that launches a thousand "where can i get more music that sounds like.." threads. besides, i dont love perdition city enough to want to see my vote go there.