My favorite album is (and I guess forever will be) Tonight's Decision, but with objective point of view I will say that best Katatonia album is Last Fair Deal Gone Down, but from another side the most interesting for listener will be Viva Emptiness album, because there is a solid story and it's perfectly shows the mood of the band. But there is another "but": new Katatonia album The Great Cold Distance is the most powerful and qualitative work of the band. So I can't even decide for myself what album is better, so I will not give you any advices. And by the way, Brave Murder Day, Dance of December Souls and Discouraged Ones are fucking classic, it's must listen to everyone.