I guess it depends on your goal or end result. If you want something very subtle, you could use the black. Having quilted a mainly black quilt with black thread, I can tell you that if is hard to keep track of your stitching because it is hard to see. If this is a practice piece and you want to improve your stitching, I would not use black, although I liked the effect on the quilt I did. Same thing with gray thread, subtle but not fun. Different colors would be much more interesting and fun to work with. Help you see what is going on with your FMQ. So, I would say #3 for this project.