I use Bottom Line, it's a strong, fine thread. I like that you can get an array of colors by purchasing one of their bobbin collections.
You might also consider what kind of stitch you're using. I use a ladder stitch with Bottom Line thread and you cannot see the stitching
at all.
You can see a picture at post #19 in this thread:
Ladder stitch for needle turn appliqué?
And an illustration of how to do the ladder stitch here:
https://www.google.com/imgres?imgurl...aBDWkQ9QEIKzAA