I have tried several, but still prefer YLI nylon monofilament, which is .003mm.
Superior has a matte finish polyester thread (MonoPoly, .004mm), but to my eye the polyester threads show up more. Here is a link to Superior's thread:
http://www.superiorthreads.com/shop/...visible+thread
My tests were for invisible machine applique. I did not test on FMQ because I never use it that way. Might be worth trying out the Superior thread for FMQ to see if it delivers the look you want.