I don't really listen to much Extreme Metal, but Opeth is by far my favorite. They have a very unique sound that perfectly blends Progressive Rock and Death Metal. Their sound has changed quite a bit over the years, but it still sounds fresh and you can still tell that it's Opeth, which isn't an easy thing to do. They're also one of the few modern (mid 90's and later) Prog bands who have taken the genre somewhere new, instead of trying to replicate the sound and atmospheres of the classic Prog bands, and this is one reason why they will always stand out in both the Prog and Metal genres. I'll talk about two of my favorite Opeth albums:
Ghost Reveries: This is my absolute favorite Opeth album, it is what I consider to be the peak of their sound. Everything that has made Opeth great is in this album and refined. The songs are epic, the acoustic passages are haunting, and the Metal parts are brutal. I've gotten chills listening to nearly every song. Watershed followed this album's sound and style, but failed to match it, and Heritage is something completely different. This is Opeth at their very best, and I don't think they will ever top it, probably makes the change of direction a good thing, but it is too early to tell. Ghost Reveries is all the great things about the Opeth albums before it culminated into one masterpiece. Most people go to Blackwater Park when picking a favorite, but I think this has everything good about BWP and then some. Spectacular album
Damnation: Much different than other Opeth releases, Damnation is the band's ode to 70's Progressive Rock. No metal, mostly clean/acoustic guitars, all clean singing, and jazzy drumming make for a very unique, but still familiar sound for Opeth. The dark and melancholy feeling that is present in past albums is still there, and the guitar solos use the same style of phrasing used in the past. This is a short album (Only about 40 minutes) but every song is strong and I'm left satisfied after it is over. It's an easy listen and very accessible, I would suggest this to anybody who likes Opeth's softer passages, but is put off by the growling or Metal.