I'd also say "Eternal soul torture" and also all of the other bonus tracks on the vinyl versions of their first three albums. I've only heard the bonus tracks once as they were so bad I always take the record off when "Tha apostle in triumph", "To bid you farewell" or "Epilogue" ends. As I mentioned in the vinyl debate I started not so long ago, the only bonus track that is decent is the cover of Iron Maiden's "Remember tomorrow", but I think the original is much, much better!
I can't really say that there's one of the bonus songs that are worse than the others, as I've only listened to them once and I think they all suck as I said.
If I have to choose from the "real" songs, I'd probably take something from "Orchid". I don't think that it has any bad songs, but it's my least favorite Opeth record, although I still think it's good. Besides the bonus tracks I can't find any bad Opeth songs.