You will get as many answers as there are quilters. It all depends on what you want to accomplish. I can not answer for hand applique, because I don't do THAT! For machine, each project may require something different. I love to do black with a satin stitch if I want the piece to look cartoonish. If I want to bring up the applique rather than the edges I use a fine thread that match the color of the piece and stitch with a very small blanket stitch. If I want it to look more rustic I use a double blanket stitch with a color determined by how much I want for it to show. Sometimes I want the edges to get more attention, and I then use a bright color or a variegated, with a big double blanket stitch. I have also done some raw edge, and I just use a straight stitch about 1/8 inch or less inside the edge of the piece, and again, the color depends on how much I want for it to show.