I voted Morningrise, but I can't believe MAYH was outvoted by Still Life. You people should be ashamed.
My vote goes to 'My Arms, your Hearse' it is a truly fantastic album. Full of atmosphere. You can practically feel the darkness and sadness echoing through the songs. It is my personal favourite from Opeth. The songs are incredibly touching. The story they tell is intricate and deep, as well as the musical and lyrical methods used to tell it are brilliant. It is a must have.
Still Life. Best album to start on your tour of Opeth-land (aside from BWP) ....but make sure you follow it up with MAYH, an altogether better album in the end...grabs you by the balls and craps a golden turd of sheer brilliance down your throat, leaving you gasping for more, MORE!!