What's the best Opeth Album and why?

Blackwater Park. I rarely Listen to it now, simply beacuse I'm sick of it to death, but it's stil their best: Akerfeldt's vocals and riffs at their peak, the best production on any opeth album, and overall amazing and diverse songwriting. 

Watershed is probably their most exciting album to date, but Still Life is still 'the' album.

I can't dig Blackwater Park as much as others may do. The second half isn't very exciting for me and I can never seem to recall anything in the title track besides that chilling acoustic part. Of course, the opener would easily fit in my top 10 Opeth Tracks.

Morningrise because it's epic and doesn't have any of those horrible clashing riffs or overly "technical" sections that make every album after it impossible to enjoy for me.

Blackwater Park because it sounds the best.

Damnation easily. If opeth did all their albums like this they would be a cool band. None of that garbage growling and no metal. But 80% of their songs are garbage. Deliverance is a poor album. Metal is boring

My favorite is Still Life.
I'd rank them like this, from best to worst: Still Life, My Arms Your Hearse, Watershed, Blackwater Park, Ghost Reveries, Morningrise, Damnation, Deliverance, Orchid.
Two reasons why Still Life is the best:
1) It is the most consistent, from a musical-quality standpoint, IMO. Every other album has at least one track that isn't quite up to the general "Opeth standard".
2) It works very well as a concept album, more so than any other Opeth album. Its lyrics read like an interesting novel, from start to finish.

I only own 6 Opeth albums and from those I'd say "Ghost Reveries" is the best, followed by "Still Life". The first one has the best track (Ghost of Perdition) and the second is the best album overall, with all songs at a great level.

I've been putting off getting anything before Still life for awhile, I guess I should jump on it. I guess Still life would be their best that I own, though I think Ghost of Perdition is their best song easily.

Blackwater park.

Cause it has the best production of all Opeth albums (thanks to mr Wilson). It might be their most brutal moment while having some of the best acoustic pieces they have done. It´s got Opeth arguably best known song (The drapery falls) and their best (The leper affinity, although thats just my opinion). It´s, along with Ghost reveries, probably the album that has the most flow to it, every song seems to marriage each other really well, as opposed to Still life, where every song is killer but they seem to individual, no connection with the next one, Blackwater has it, and it has it better than in any other Opeth album. All songs are incredibly composed and they are quite varied within each other, in tempos primarly, which is another thing Still Life doesnt have. All songs, as said before are great, but it´s never really "on", it never really get´s fast and heavy, it seems a bit too lineal at times (probably due to the production), Blackwater park doesn´t, it´s a mountain ride that doesn´t let go. And Wilson sings in one song... what more can you ask for?

Ghost Reveries has such a tremendous set of riffs, memorable passages, melodies, guitar solos, etc., great atmosphere, and great flow, among other things.