Which Katatonia album to get- Help me out please

It depends on what you have heard of them, and what you like of it, ofcourse.... You're the one to answer that question.

My advice is to start with "Dance of December Souls". It's a really good album and you get to follow their timeline, which is pretty exiting.
 
I suppose the best course of action would be to weigh the material you have heard from the band and consider which song effects you the most. Find out what release this song is included on, and make your purchase. Personally, I usually begin with a band's most recent material, and judging on my response to that material, I backtrack accordingly.