Hard to say, as all of their albums sound quite different. The Bastard has Black Metal influences on them, whether it's in the tremolo riffings or the Black Metalesque vocals that appear once in a while. It's an epic concept album and a very interesting debut. The August Engine is probably my favourite, because it's quite eclectic in having some Thrash, Folk, Trad. Doom and even Gothic (but not gay) influences.. From The Locust years and on they went more for a 70's prog vibe, with lots of organ and stuff. All of these albums will take some plays to grow, as all good Prog albums should. They're very rewarding on multiple listenings imo.
Cathedral - Aphrodite's Winter