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.