My absolutely favorite machine quilting thread is Valdani. It's manufactured in Canada and their variegated 35wt - 2000m cones are the best. The thicker 35wt stands out so well on a quilt.
Go to www.valdani.com to take a look. They also have some good deals on the large cones. I just ordered 2 of my favorite 2000m cones for $5.80 each.
I use the Coats 50wt machine quilting mercerized cotton thread for piecing and some of my machine quilting.