I realized that it's hard to pick the "best" opeth album because all of them have their own aspects of greatness
orchid- mysterious, beautiful atmosphere, gorgeous lead guitar melodies
morningrise- outstanding classical acoustic guitar parts, heartfelt clean vocals
mayh- sorrowful mood throughout, great flow, depressing and dark
still life- great songwriting, acoustics, plays out like a classic movie
blackwater park- crystal clear production, very mature, beautiful autumn/winter feeling to it
deliverance- complex, genius arrangements, haunting lyrics and death metal vox, brutal yet beautiful
damnation- (obviously) softer side, amazing playing from every member, very creative
ghost reveries- the most mature opeth album, most songs are finely crafted and spellbinding, keyboard adds a new dimension to the heavy sound