I believe it was E V I L who once concluded (I'm loosely paraphrasing) an axoim that if there was something about Opeth that didn't strike you as being brilliant, you just hadn't "gotten" it yet.
In that case, BWP would be the hardest to get into, 'cause to this day I don't feel it is as good as the other four Opeth albums, and thus, I haven't "gotten" it yet.
If this is NOT the case, then the answer would be MAYH. It was the first Opeth album I bought, and a year and a half later I suddenly realized it was the best Opeth album (and, by implication, the best ever, as my previous "favourite album ever" was Morningrise), seemingly out of nowhere. I was just listening to it, and I suddenly realized "Hey... waaaaitasecond..."