If there was a movie based off an Opeth concept album, I'd say it would have to be MAYH. Even though I think Still Life is a superior concept, I never REALLY associated Opeth with that much gore. If they were to pull off Still Life, it would have to be more artfully done and I don't really see how that could be pulled off in an Opeth manner (just MO). As for Ghost Reveries, the concept wasn't fully expressed (I think more of the album didn't tie on to a concept than just Isolation Years). So that leavs MAYH, which would make an awesome movie.
Now if we're talking about one of those album-long music movies like The Wall, I'd want it to be Blackwater Park.