Impossible to say one, but at this moment it's propably Burzum - Det Som En Gang Var or some early Mayhem stuff. It changes all the time.
Well, Stein um Stein is a great song with hilarious lyrics and is built really well, but it also lacks the symphonic sound that I love in MHB.
BTW their single for the new album sucks donkey balls. Someone should explain Lindemann that his English accent is shit, and that "you have the pussy, I have the dick" doesn't sound witty, or even controvershul.