I got three sets of BOBs at Superior Threads that are put together for hand applique. The thread is Bottom Line, made for longarm quilting, but it is a VERY fine thread. It's probably comparable to a silk, though I haven't tried that yet. I like it a lot. It is strong and color matching just makes it disappear.
For machine applique I have been using Sulky monofilament on the top and a thread that matches the background on the bobbin. Can hardly see it when I use a tiny zig-zag.