For a person that hasn't listened to Opeth all that much, Blackwater Park, followed by Still Life and Deliverance, is easily the most accessible. The others are good albums for sure (I like the newest stuff best but notice I'm not recommending that to just anyone), but take some listening to really get into. I think MAYH is certainly a fantastic concept album with good flow, but imo the individual songs are a little boring compared to the much more complex, well produced, and interesting instrumentation of the newer stuff. I would absolutely NOT get MAYH first unless you know you have a history of getting into the intricate stories of concept albums. If you're looking at MAYH because people have told you its good for heaviness with less clean vocals, then fuck it and go Deliverance for the same reason but a more accessible album.