I'll need to get around to Orchid, but I know what I'm doing with Morningrise.
First of all: Eternal Soul Torture definitely needs to be promoted.
My first elimination is Nectar. It's the kind of song that I enjoy on the spot but don't forget everything about later. It's still a good song though.
Second vote is for To Bid You Farewell. Again, great song, especially as far as acoustic Opeth goes, but I like the more complex songs here.
Speaking of complex, BRI would be my third pick. It has a stupid amount of parts and it'll take me many more listens to realize it fully. I sometimes feel it doesn't flow too well, or rather when I think about the song it just feels like there's too much there, I guess. There's this sick melody at 13:37 (probably my favourite section) and it totally sounds like the song ends there. But then it goes on for another (good) 6 minutes.
I wasn't a fan of The Night and the Silent Water on first listen, but it came to be my second favourite here. The dual guitars in the verses are so good. And there's this smooth transition to the second melody, at 2:11 with the "whispers..." and the ride cymbal. The acoustic section is pretty good too and I like the buildup, though the singing of the last lyrics is kind of awkward and it's also quite lengthy.
And in between the complex BRI and the more streamlined TNATSW, Advent gets it perfectly. It kicks off immediately with this killer, almost power metal riff. Switches for a short time to this chill bass part, then back to another riff and finally around 2 minutes in we get to the verse with both a double bass and really cool bass. I love the rough screams, especially on the "trying to adapt to the wilderness" line. There are some cool bits with muted clean guitar. There's a lengthy acoustic part in the middle which shifts with this classical thing to a heavy bridge, and finally after all this back and forth (which I believe I missed some of) it morphs into an even heavier section. The bass solo there is kind of weird, and the ending is maybe long. But this song really has everything.
I think it's very different to the later albums, most obviously the guitars. The dual guitar style on Orchid and here is absent on everything after MAYH. On MAYH it still appears, but a lot less and mixed with the chord stuff that they used on Still Life and after. The bass is a lot more audible which I like. Opeth always had good basslines but 1) they're harder to hear on later albums and 2) Morningrise easily has the best bass performance here.