Yeah the fact that there's a pretty equal distribution, with each Opeth album capable of being regarded as their best, is probably as good an indicator as any what a damn good band they are. Orchid probably ties with Blackwater Park as my favourite, Morningrise would be as good as Orchid but I felt it had too much flab (particularly the last 8 minutes or so of Black Rose Immortal). On My Arms it felt like they were still refining the sound they fully realised on Still Life and Blackwater Park.
As for the new album, I just hope it's better written than Ghost Reveries. I thought they were too preoccupied with the new keyboards, they forgot to write good songs. It's the only Opeth album with lots of filler ('Atonement', 'Harlequin Forest' and that piece of shit 'The Grand Conjuration'). So I have high hopes for the new album, in one sense, because hopefully Mikael will write better stuff. But on the other hand, it's basically a whole new band he's working with.