My Arms, Your Hearse gets the vote for my favorite. I think the album had the best lyrics, songwriting, production and atmosphere out of any album they've done.
Blackwater Park would be a close second- it was the first album I heard from Opeth and the one that got me into the band. I especially like the 2-disc special edition- I think disc 2 has two of the best soft songs Opeth has ever done.