If you know how to adjust tension for your machine, the answer to “which thread?” is “all of them.” I have used everything from invisible to 60 weight to 50 to 40 to 30, 2 ply, 3 ply, metallic, poly, cotton, and poly/cotton. They all work; it is just a matter of what look you want. Always adjust your bobbin tension first, then adjust the top tension.