I like 1.5 for paper piecing (easier to rip off the paper). For other piecing, it varies between 1.5 (unsure of fabric quality, know the seam will be cut later, or know the fabric likes to fray), 1.8 (default for stitching with my quarter inch foot on my Janome), and 2/2.4 (I was sewing clothing and forgot to change it).