That said, The Night and the Silent Water. I can really relate to that song... my grandfather past away about a year ago, and it seems Mikael felt the same way about his grandfather's passing as I do about mine.
I'd say "Wreath", too, because it's just BREEEWWWTAL
Also some songs from MAYH are really really heavy I think (April Ethereal, The Amen Corner, Karma).
But each of us has another opinion of the word "heavy"...
I gotta agree with Wymrwood on that one. Godhead's Lament has one of the heaviest opening riffs I've ever heard Opeth play. Heir Apparent's def in there, as well as Baying of the Hounds.
Guitar-wise: Baying of the Hounds, Blackwater Park, Heir Apparent
Vocally: Blackwater Park, Heir Apparent, Demon of the Fall, The Grand Conjuration, Wreath
Lyrically: Wreath, Karma, Blackwater Park, Serenity Painted Death, maybe The Leper Affinity