I have used nylon monofilament thread (.003mm) for applique a lot. It is the most invisible thread I have found for turned-edge machine applique with a small hem stitch. There are some tricks to using it without problems, such as lowering the top tension on the machine, that you would want to know before using it. You also need to use a good brand of this thread, as off-brands can have problems. I like YLI nylon monofilament.
For a quilt that will be used a lot and washed frequently, however, I would use a 100wt polyester thread. Nylon thread is simply not as nice to touch as polyester thread. I would use 100wt polyester rather then silk thread because silk, being a natural fiber, can deteriorate under adverse circumstances (such as lots of washing, harsh detergents, exposure to light, etc.). Of the 3 types of thread -- nylon monofilament, polyester, and silk -- polyester will last the longest, yet still feels very nice. Nylon thread can possibly become brittle over time (although this may no longer be a problem due to newer manufacturing techniques; hard to be sure at this time), probably due to exposure to light, and polyester thread does not have this problem at all, so again I think polyester thread is safer.
Rayon needs to be avoided, as it will not hold up to abrasion (as in a washing machine or dryer) at all.
Last edited by Prism99; 05-20-2017 at 12:27 PM.