If you want your thread to disappear and not show, it is best to use a fine thread. There are many good ones out there. Superior's Masterpiece, Mettler's 60/2, Aurifil's 50 wt., these are some of the best. Superior makes the "Frosted donut" which is a set of 36 bobbins of all their colors, great for applique, when you need lots of colors and don't want to buy a whole spool of many colors.