"A Blaze in the Northern Sky", "Under a Funeral Moon" and "Transylvanian Hunger" are the three essentials imo. The rest up to and including "The Cult Is Alive" is excellent and among the better black metal of the 90/00s (with "Sardonic Wrath" and "Ravishing Grimness" being the weakest of the bunch). I liked the change they did with "F.O.A.D.", which turned out to be a fun little record. The next two didn't do much for me, but I got really into "The Underground Resistance".