Cool thread, as I've always thought the intros have been one of the strongest Opeth traits.
I would originally have said The Moor, but these days Prologue/April Ethereal. That said, the beginning of Advent is awesome, the beginning of The Leper Affinity is morbidly exquisite, not a fan of Wreath's brutality (but the beauty lies in that it is so different from the others) and Ghost of Perdition slays.