Beautiful quilt top! I love the colors and the symmetry. A great scrap-buster, for sure.
I love complex machine quilting, so I would do something really challenging. Some ideas: stitch in the ditch of every smaller square, then do a four-petal flower inside each square. In the medium squares, do the same thing as far as outlining it, maybe echoing that outline inside the square, so it has a double outline 1/4-inch apart, and then do a six-petal flower in those squares. In the large squares, do the double outline, but with the lines 1/2-inch apart, and then do a "double flower" like the one in this blog:
http://christaquilts.com/2013/04/24/...ouble-flowers/
I might leave the white unquilted, to highlight the squares and flowers. I'm not sure. I might also do some sort of dense background quilting in the white.