I think "Ghost Reveries" is the most polished version of their "classic" sound. Wonderful album. I also love "Watershed", though many people dismiss it because it is a departure from their older style. It's definitely a weird album, but it's really quite good.
Out of their pre-"Ghost Reveries" albums, I think "Blackwater Park" is their best all-rounder, though I absolutely love songs from anything from "Still Life" through "Damnation" (never have been able to get into their first two albums).
"Heritage" just leaked, and it MIGHT be Akerfeldt's best work yet. Yeah, it's not metal, but it's awesome. It can be chill, heavy, intense, fun, or any combination of those (and more). Very diverse song selection. It takes a few listens to "get", but once you do, it's amazing. Akerfeldt has always said this is the type of album he's wanted to write since he was 19, and you can tell by listening to it. It definitely has a 70s prog rock, jazz, and...well, Opeth vibe to it. Haters are gonna hate it, but if you appreciate music, you'll like this. Sometimes bands change their sound. Musicians get bored writing the same stuff over and over, and I'm happy to support their desire to play something different. Sometimes it doesn't always work out well, but "Heritage" is absolutely fantastic.
It comes out on the 20th, by the way. I recommend you buy that next. It might be beneficial to listen to "Ghost Reveries", "Watershed", and then "Heritage" in that order (listen to them each enough times to really "get" them before moving on). It makes more sense if you listen to the progression of sound, though I do think almost anyone could get into "Heritage".