let`s see....
when opeth begins, there was a mix between black metal with progressive 70's rock ands some heavy metal influences (orchid and morningrise). (my favourite era of opeth)
from orchid the insights are: forest of october (my favourite song of opeth), in mist she was standing, under the weeping moon.
morningrise maybe is the best album of opeth. all his songs are great (maybe nectar be less great than others, but is great too). the ballad to bid you farewell is amazing, and black rose immortal is a amazing black metal song.
MAYH is less black metal, more closer to death metal with the same influence of prog music. (really good too) the best songs to me: april ethereal, when, demon of the fall, credence.
Still life was a predictible step in this evolution. I think the true essence of Opeth ended here, his last really amazing album. the best songs to me: the moor (the best of the album), face of melinda, white cluster.
BWP was a suprise to me, less strong than any other album with a strange new influence (i'm not sure if is steve wilson), i don't like it and i feel really dissapointed of it.
Deliverance was for me the next: "ok, they don't will do music like they did before, but this new sound likes me". a really good album, with a new kind of power, more closer to death metal (master`s apprentices, by the pain i see in others). a brutal album. maybe deliverance is " Morningrise of new millenium".
Damnation is a great album, thinkin' in isn't a heavy album. a jazz influence is very notorious in all songs, and Pink Floyd too

DD (In my time of need, to rid the disease, death whispered a lullaby, etc)
GR finally, is better than BWP to me, but weak besides deliverance. is the first time to me no one song has a really great clean part. i don´t like the keyboard on opeth. is more progressive than any album before but personally i don´t like it. only 2 songs are average (ghost of perdition, the baying of the hounds), others, less good.
to the people who knew opeth with GR, obviously they think is the best album, but i wanna know what are thinkin' the old fans of opeth about it.
i respect the evolution of the bands, i hope opeth releases in near future really brutal albums, but the road who has taken ...i don't like it