Ghost Reveries is probably the best way for a newcomer to get into Opeth but it hasn't aged well for me. I'm not sure if it'd even be in my top five Opeth albums at this point. I dunno, something about the sound of that album just feels very... flat. And the pacing is pretty bad imo; after the great start that is the first three songs the album just gradually gets less and less engaging.
I know the fanbase is pretty divided on Watershed but I think that album improved upon Ghost Reveries greatly. More variety, more energetic performances, and more weirdness (in a good way). Less metal I suppose, but Heir Apparent and The Lotus Eater more than hit that quota for me.
I never really got the
Ghost Reveries thing, to be honest. I get it being accessible because it's sort of the median of their sound—a lot of both metal and prog-rock-y parts, a lot of both clean and harsh vocals, etc. But it's never really connected with me, even after giving it many chances.
Granted, I don't exactly consider myself an Opeth fan. Music with a really bleak (
) vibe isn't for me, and I'm made uncomfortable by the fascination with murder that comes across in a lot of their early lyrics. But I like
Blackwater Park a lot better than
Ghost Reveries, even though BWP is a lot darker in my judgment.
Watershed, on the other hand, is my favorite metal Opeth album for a lot of the reasons you name. And lest I be accused of just saying that because I'm not enthusiastic about metal Opeth, Heir Apparent is my favorite song on that album, and way better IMO than any song on GR.