I am surprised Damnation dominates this poll so much. I speculate it's, at least partially, due to it belonging to the death-metal era of Opeth, despite being a clean vocals album. Sure, it is a collection of nice songs but it's also kind of boring, unless you are in a rather specific mood.
I voted for In Cauda Venenum - to me, it feels like they finally found their own original brand of prog. I like Pale Communion quite a lot but it does sound much closer to generic 70's prog.
I prefer death metal era Opeth but I would definitely rank their best clean vocal albums above their early stuff and, maybe, Watershed and Deliverance.