I quickly gave up even attempting to find bands like Opeth, because I know none exist. IMO, they are unique - I listen to their music, and it's just on another level. Sure, you can pull their music apart into a million pieces, and then compare each piece with another band, but at the song level, I find it impossible to do with Opeth.
Maybe in the broadest of spectrums, you could categorize them with others, but I think they would still distance themselves from others. Uniqueness is a quality very few bands have, and Opeth is one of them.
At this particular moment, I am struggling with naming other unique bands.