Ghost Reveries is my favorite Opeth album, so I would have to say get it. It definitely has the best sound quality of the three, that's indisputable. But, of course, depending on your own tastes, one of the others may be a better choice. If you like more constant heaviness and shouting/growling/screaming, then go with either Orchid or My Arms Your Hearse, but if you like more melody and variety, then go with Ghost Reveries (again, my personal favorite). As someone else has already said, though, you're probably going to get them all anyway, so it's probably not that big of a deal....