Wow, I really disagree with that list.
I'd say go for Coma of Souls... German thrash at its best
. At least Steve Wilson didn't taint that record
.
No, but seriously, your logical progression is Blackwater Park. Out of the rest of the material, its the one that sounds most like MAYH and Still Life, despite overall being simpler and less atmospheric. It was sort of like a production/musical experimentation playground for the band and their newfound producer.
Stay the hell away from Deliverance, or at least listen to it at the end, so that you only get doubts about Opeth's credibility after you've absorbed the magnitude of earlier opuses. So after BWP, I'd say go for Orchid or Morningrise. I personally don't like Morningrise much. It sounds like a more monotonous, boring and longer drawn-out version of Orchid, with less guitar solos, and more bass wanking.