Melissa and Don't Break the Oath are both so fucking brilliant and perfect. I got into Mercyful Fate relatively late compared to most of the other big names, but those two albums have done nothing but grow. Most prog metal bands wish they had songwriting that involved, let alone their contemporaries in trad metal. Even stuff like classic Maiden and Priest I've begun to see some of the relative weak points, but those two seem bulletproof. I think I prefer Melissa for its higher consistency both in writing and in tone/mood, and because it has the single greatest collection of riffs, Satan's Fall, of basically any song, but Don't Break the Oath has sharper left turns and probably more 10/10 songs.