It all depends in the definition of you give to the word Heavy when referring to a song. For some people, Heavy means brutally fast and aggressive like Napalm Death or Krisiun, for some others, Heavy means downtuned and very slow like Solitude Aeturnus or Cathedral. Some others will say that Heavy is rather a compromise between the aforementionned, meaning downtuned and brutal like Obituary or My Dying Bride.
There is also the lyrics content which could turn a ordinary song into a heavy song or vice versa.
For Demon of the Fall and Master Apprentices, I don't consider them as 100% Heavy because they start Heavy but shift slowly into more melodic songs.
Run Away ... Run Away ... is not as Heavy as Morbid Angel's Blessed are the Sick if you know what I mean.
Anyway, back to the topic. The three heavieast Opeth songs per my opinion are (by coincidence) in exact chronological order in their entire discography.
1- Blackwater Park
2- Wreath
3- Deliverance
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