I'm always flipping between Morningrise and My Arms, Your Hearse as my favorite Opeth album. None of their other albums, particularly their last three, even come close to the cold atmosphere on these two albums. Still Life comes pretty damn close, but the guitars sound a little thin on this album. And I used to love Blackwater Park, but I haven't had the urge to listen to it in well over a year now. But I know that album has a pretty dark atmosphere as well. My ranking:
1. Morningrise
2. My Arms, Your Hearse
3. Still Life
4. Orchid
5. Blackwater Park
6. Ghost Reveries
7. Damnation
8. Deliverance