This is an interesting question. When I read the title of the thread, Hostage To Heaven by Grip Inc first came to mind, then Stjerneheimen by Enslaved.
Then when I opened the thread, I saw this was about your favorite Opeth guitar solo. Funny, but I never think of Opeth when people ask me what my favorite solos are - guitar, drum, bass, keyboard. There music, the band, the entire package always seem larger to me then at a solo level. I always consider them as one - four integral parts making up a whole. With so many other bands, you key in on one person, whether it be for their ability with their instrument/voice, their stage presence, or their star status.
But still giving the question some thought, I'll go with The Funeral Portrait - although there are so many Opeth places that constitute favorites.