I've gone through various stages of Opeth album preference and after a few years of listening to all of them I'd probably have to say
Orchid: Pretty eerie and organic. Interesting at first but it becomes a bit of a drag to listen to eventually as the songs kind of meander around sometimes without much exitement, and make me think ... tbh, i'd rather be listening to a different album.
Very Atmospheric though.
Morningrise: Was my favourite album of theirs to begin with, has a lovely clean sound where all instruments can be heard significantly. The songs are as long as, and if not longer than Orchid however I generally find them more interesting and atmospheric, over time though I found other albums more exciting and captivating.
My Arms, Your Hearse: This really is their best album, it's short, doesn't **** around and just puts out 48 odd minutes of excellent metal with epic memorable tracks that really get you going. It is however a bit dodgy sound quality wise, non-present bass guitar almost.
Still Life: Arguably better than MAYH in a lot of cases, containing more variety of music and more complexity, as well as improved vocals and the arrival of new bass player Martin Mendez (although you still can't really hear the bass under the music). This is where the catchier tracks start coming in for Opeth, but they're very good and well placed.
Blackwater Park: This one is a lot of people's favorite however it never went down well with me. This time they use a more mainstream sounding production, some of the songs really annoy me (Bleak IMO is their worst track). There's a couple of good songs but overall I thought it was a very weak album in comparison to their past ones. I don't really see how it's catchier than Still Life either.
Deliverance: Has some good tracks and great display of technical drumming, however overall I felt it was quite inconsistent and the band themselves even admit this due to their trouble with the studios etc etc
Damnation: I actually really liked this, and I still do but after hearing Green Carnation's The Acoustic Verses this album feels very meh in comparison. It's a very easy listening CD that fits a lot of moods so it's got quite a lot of playtime for me. It's an intelligent mix of soft music with very dark lyrics, overall song quality is pretty consistent, although the "fun" song Ending Credits doesn't really fit the overall feeling of the album.
Ghost Reveries: I really loved this after hearing it as it was the second Opeth CD I bought, but after hearing all of their old work this album doesn't really make sense to me. It's got lovely sound quality and some fantastic drumming by Martin Lopez (his last album unfortunately) however the band starts overusing the mellotron and starts becoming very pretentious with a lot of their songwriting, and new influences are clearly present (Modern Day "Prog" Rock such as Tool and Radiohead etc) as well which I feel is starting to wreck the great atmosphere that their past albums have conveyed and don't quite convey the emotions as significantly as the past ones. I find I love bits of the songs, but then the very same song will have a few bits that make me roll my eyes at the wankery.
Watershed: this is where I stop listening to the band, they've taken too many steps in the opposite direction of what I enjoy about their music and I never really gave this CD a chance but I don't enjoy it at all. Pass for me.
YMMV for all of my input, but if I was to name essential Opeth albums I'd probably tell you to get Morningrise, MAYH and Still Life and perhaps Damnation, but for newbies who are still coming out of the Hardcore, Rock or Nu-Metal stage of diving into Metal you would probably enjoy their most recent two albums the most (at first anyway
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