I use Bottom Line in all my bobbins. It's 60 wt, so you get more thread on a bobbin and don't have to change it as often. It comes in quite a few colors, but you can start with black, white and several neutral shades. It's from Superior and I buy the large cones. I usually use it with King Tut on the top, but other top threads should work. It really works well. I have a Hinterberg winder that I use for bobbins. It gives a nice tight wind, which is what you want on a long arm.