Oh boy, this is a great quilt!
I think the blue-grey from the background of the centre block (the red star) would be my first choice. It's a bit darker than the shadow, so would hold it all together visually.
Second choice would be the shadow fabric.
I really wanted to suggest red, since there are lots of reds in the blocks. However, I think that would draw too much attention away from the piecing, so I'm going to say not red. I think that a pieced binding might have that same problem.
I can't wait to see what it looks like with whatever you choose!