I used to use Bottom Line, but had weird tension issues so I switched to Superior's So Fine a couple of years ago and use for both bobbin and top thread. It is a fine poly thread, but feels and looks like cotton, really sinks into the quilt for machine quilting on my longarm, and seems to hold up better than Bottom Line if a quilt is going to be wrangled, wrapped in, or slept upon. So Fine is my favorite for all of my piecing as well!