Some times I just use a matching or a contrasting thread, almost always the 30 wt Madeira Rayon. I like the colors and how it sits on the surface of the work.
But sometimes I like to use heavier weight threads in the bobbin to get a more decorative look to the top stitching.
I have found that I can put all kinds of heavy thread in the bobbin and sew from the backside of the quilt and end up with a very unique and different finish to the top stitching.
In this video I am using the 4mm silk ribbon that I dye on the edge of Woodland Treasures.