I enjoy all of Opeth's albums (obviously), but my favorites are their earlier albums such as Morningrise and My Arms, Your Hearse. I was introduced to them after purchasing Deliverance, Blackwater Park and Damnation, so those will all always have a special place, but I think I enjoy their first three cds the most, at least at this point in time. My least favorite of the seven is Damnation, I rarely listen to it, especially in comparison to any of their other albums.
Rankings excluding Orchid, which I don't feel I have listened to enough at this point to include.
1. Morningrise
2. My Arms, Your Hearse
3. Deliverance
4. Still Life
5. Blackwater Park
6. Damnation
Orchid will likely move into that number three spot and move the others down.