My absolute favorite of the planetary story arcs is Voss. But then, I'm a huge fan of "Heart of Darkness", which this story was loosely based on.
I loved everything about Voss. The "trail" was awesome, it took me 5 tires to get that last boss. The twisted forest really looked creepy.
I think the Voss people are coolest looking in all the galaxy. I wish they were a playable species.