Everyone has varied opinions on Katatonia's albums. I think an interesting thing to do is check out individual songs from all of the albums (here is where P2P programs,or friends, can help bands) and decide which is the most appealing. Chances are you'll have a hard time deciding and eventually get them all...you need a starting point though. The most opeth-like is Brave Murder Day, but it's not a good representation of their current style. Some songs to check out
Nowhere, Brave, Sleeper, I Break, Deadhouse, Stalemate, Criminals, Evidence, Black Session, Teargas, gateways of barevement, endtime, tonight's music, for my demons that should be good for now