I think their most shreddy solos are definately on The Funeral Portrait; I love that song. To be honest, not alot of Opeth's stuff is technically demanding. What is hard to nail, for me anyhow, is the the right tonality and vibrato width and such when it comes to Mike's soloing... there's just something about the way he plays that only comes with having played as long as he has.
The album that comes closest to being technically demanding is definately Still Life. Not just the solos but alot of the riffs in general take a greater degree of precision to nail, simply because alot of them are so damned 'weird' that you would never have thought of playing that certain series of notes or chords.
Btw, yeah Mordet i Grottan shreds pretty damn hard, that solo owns too. One thing I think is that they should've sped up the solos in The Funeral Potrait some more, as those things are just made to go hard.
What people seem to forget alot of the time is that there is two very distinct, different and contrasting ends of the soloing scale. You have the 'feeling' side which is meant to enhance the general feel of the song and is more of an extension of a lead part as opposed to being a 'solo' per-se. I favour those. The other end is the end which is technical for the sake of being technical and believe me that in itself can add immense enjoyment to even the most 'emotional' of songs. It is simply different styles and different forms of appreciation.