1 Deliverance (2002) Every song is prime Opeth, peak of their maturity, and I'll never forget watching the title track live in Bell Center, Montreal.
2 Blackwater Park (2001) Nearly prime Opeth not quite there yet: some parts are mediocre, too long, some filler here and there... but the album that made them more than an overly romantic melodeath prog band and made them "Opeth" as we now know them.
3 Ghost Reveries (2005) feeding off the momentum of the magnum opus Deliverance, parts are still inspired, well written, but a cleaner smoother production.
4 Damnation (2003), some very catchy clean tunes, good all around level, ripe song-writing.
5 Morningrise (1996) has the beautiful Black Rose Immortal which doesn't feel like 20min. Some nice parts on the record besides that one too.
6 Watershed (2008) fairly underwhelming effort overall, went too blockbuster/big production... but the track Heir Apparent is superb.
7 Heritage (2011): an all clean singing prog rock Opeth album ? Concept's fine, but not the execution. Fairly uninspired and uninteresting generally. The "GOD IS DEAAAAAAD" song is excellent though.