ooh, missed this one the first time around, so I must compensate by overly-participating now. Yay for 4-day weekends.
First, we have to establish what separates 'Morningrise' and 'Orchid' (the good Opeth) from everything that came after (the shitty Opeth), and 99% of other metal bands. It's the fact that the guitars almost exclusively play winding, single-note-at-a-time, mid-paced, repetitive melodies throughout the songs. Gothenburg-style melodic death metal bands do something close to this, but their tempos are different and the rhythm section doesn't work at the comfortable walking-through-the-woods pace that Opeth does. A good number of folk/Viking/black-metal bands are also close, but then they usually cock it up by adding some keyboard nonsense. There's something special about carrying the melodies on a guitar, particularly if it's that dry, creaking lead guitar tone on those two Opeth albums.
So with that established.... (play video for clips of all the mentioned albums)
I still think Ulver invented the whole Opeth style in the last two minutes of the first song on 'Bergtatt'. (Anathema also does an exact version of the style in "A Dying Wish" from 'The Silent Enigma', but that doesn't predate Opeth).
Then from the same place/era we get Ophthalamia's 'Via Dolorosa'. Their connection was already discussed quite hilariously
here: Zod said Ophthalamia sounded like an Opeth cover band, based on their name alone. Raggi misunderstood, thinking he was referring to the sound, and called Zod stupid. But Zod was right! Even though he had never heard the album. The entirety of 'Via Dolorosa' is a winding guitar melody. It has an almost identical tone, pacing, and repetitiveness of the Opeth guitars. The main difference is that there is only a single guitar, as opposed to Opeth's intertwining pair, so it's not nearly as interesting.
The next obvious place to look is Katatonia, since Akerfeldt did the vocals on 'Brave Murder Day'. That album definitely has lots of lead melodies, with "Endtime" sounding particularly Opeth-ian, but some of the rest feels a bit more like "sitting alone in my bathroom" than "walking through the forest". So then the next place to look is October Tide, the Katatonia offshoot. Even though Akerfeldt isn't involved, and the songs are relatively short, it's actually a lot closer to the Opeth style overall.
Since October Tide has jumped us forward in time, that leads us to Agalloch's 'Pale Folklore'. Their guitars sometimes don't stand out in front of the mix as much, but they've certainly got the forest-feel down, and they also mix in the acoustic stuff well. Then of course you can't think of Agalloch without thinking of Sculptured's first album. Certainly the trumpet isn't very Opethian, and the clean vocals are a rather different thing, but the whole thing is melodic-death-that's-not-Gothenburg, so that puts it somewhere in the same ballpark.
The gray-metal of Agalloch leads to Blazing Eternity's 'Time And Unknown Waters'. Sometimes they tend a bit more towards the goth-y side, but the pacing is right.
Finally we come to Novembre, who, despite having broad similarities to early Opeth for much of their career, didn't really make an album that closely followed the rules until 'Novembrine Waltz' (which is probably why it's their best one).
The only thing I don't understand is why there haven't been more early-Opeth clones. The best explanation I can come up with is that people who are smart enough to recognize the genius of early-Opeth have too much pride to simply rip off another band. Or, maybe it's just hard to do.
Neil