I don't think anyone here really deifies Opeth in the fanatical sense to which you refer. (Not that I know of anyway). This is why. If Opeth were deified here, everything they do would be considered equally godsent and magnificent, (granted, there isn't a song I don't by Opeth), and there could be no favorites, since even making the suggestion that anything God Emperor Opeth does is less good than anything existant would be blasphemy in our eyes. Conversely, all of us have our favorite albums, least favorite albums, (BWP in my case), favorite songs, least favorite songs (demon of the fall in my case). Also, despite the fact that "Mikael is God" is that person's handle; said person has already expressed that they do not really believe in a God, (if I remember accurately, correct me if I'm wrong), or something to that effect.
I think, also, that anyone who labels himself an "Opethian" loves the essense of that which makes Opeth, Opeth. So, until Opeth begin to take on different qualities, and become a new entity altogether, (e.g. Metallica), the people who love their music will love all their music, because that is the essence which they are in love with.
Speaking for myself, I don't regard them so highly for no reason. In fact, each album I got by them, I rather disliked for a while until about 10 listens or so. But now, the emotion I derived from their music, (especially Morningrise), is unparalleled by any other feeling from any other band. For this reason, I call Opeth my favorite band, and Morningrise my favorite album, so far. If I find a band I grow to love even more than Opeth, I don't think I'll have a problem admitting it. In fact, I'd love to see the day, (that would be some damn good music!), though I doubt I will see it anytime soon, because Opeth are so different from any other band. It's a sound that exists only in Opeth.
I don't attempt to call them the greatest band ever, because I haven't heard all the bands that are, were, or will be; so I can't make that judgment. But I can say that Mikael (with Peter) is the greatest songwriter of music that I love that I have ever heard. It's all opinion-based really.
IMHO, if you really want to find a songwriter you can call the greatest ever, it would probably be a classical composer.