I have tried the YLI size 100 silk, Aurifil size 50 and 60, Coats Dual Duty, Bottom Line, YLI Invisible in clear and smoke and So Fine.
For me, the Bottom Line wins - a light and medium warm neutral and a light and medium cool neutral seem to work for almost all the fabrics I've been using in applique.
The thread is easy to handle and doesn't shred or tangle as easily as the silk or the finer cotton threads. It's easy to knot and tie.
It has no shine on the applique and it won't melt under the iron. It is completely invisible where I've sewn small enough stitches.
I already loved the stuff for quilting, so now I'm really hooked. :)
Bottom Line on bobbins - 12 colors per set:
http://www.superiorthreads.com/shop/...ique/products/ I bought these sets to cover those situations where the neutrals aren't disappearing enough. (But, I now think if I had just added a dark warm and a dark cool neutral, I might never have had to use the colors that actually match the appliques.)
Bottom Line on bobbins - 35 colors in a Bobbin Saver ring:
http://www.superiorthreads.com/shop/...bs-2/products/
Bottom Line on 1420yd spools:
http://www.superiorthreads.com/shop/...bs-2/products/ - if you get silver, cream, taupe and grey, these four colors will disappear on most appliqued fabrics.