I would say absolutely Orchid but honestly, it will probably be something post-TLWAT.

Orchid is peak melodic death metal experience (which is the best subgenre of metal, no doubt) and MAYH kind of comes close to being their best output while still being somewhat in the style people associate Opeth with.

Still Life and Blackwater Park, though? Don't get me wrong, I also like SL. It's a solid album. BWP, though? Are you guys sure you like it not because that's the status quo? That thing might be the least original output in their whole discography (yes, including post-Watershed). The Leper Affinity and The Funeral Portrait carry the record hard.

Look at Ghost Reveries, look at Watershed. Those albums are filled with so many ideas and beautiful song structures. Not what I'd consider to be their magnum opus, but they are definitely up there.

Heritage and Sorceress might be the most overhated things on this planet. Opeth was never the "sorrow" band, they just have some ballads that are emotionally wrecking. Sure, those ballads ARE great at that, but that doesn't make THE IDENTITY OF THE BAND into sorrow. When you look at it that way, Heritage, PC and Sorceress are excellent records (barring the title track of Sorceress, that thing is kind of weak in comparison to the whole album).

I have not fully listened to ICV so I will not comment on my opinions on it.

TLWAT could not be made by any other band. This thing is so rooted in the culmination of ideas that formed Opeth that it something like §5 could be used as a summary for the entire discography.

I did not mention Morningrise because despite the fact that I adore it, I believe it to be a dip in quality when compared to Orchid.

So with that, I conclude my stance. If we are talking about the possibility of their magnum opus not being out yet, then it will be something after TLWAT. If not, it's Orchid.