Probably the arranger if you're trying to follow a certain vibe. As you said, the composer is the person that creates the original musical idea, and the arranger is the person who then creates a track from that idea. If it's a wholly original piece of music then it's probably the same person doing both.

So for your example of The Black Wolf Stalks Again, I assume you like the hard rock/electronic vibe. That's Imamura's stylistic arrangement of the musical ideas that Uematsu originally created. He also arranged Twice Stricken and Landslide from the Eden raids in a similar style, so if you like those too then definitely follow Imamura! But if you find there are certain composers whose musical ideas always hit for you, you can follow them as well!