i use silk thread for applique (and most hand piecing) it is very fine- smooth- a joy to work with and very strong.
i've heard lots of do's and don'ts...some people say you should not use it-
i've been using it for many years now- and have never had any of the problems i have read about.
any strong thread will work- you want to match the thread to the appliques- or use a neutral (like light gray for light fabrics, medium-dark gray for darker threads)

I was taught to use Aurofil; it is two ply and has alot of colors including the ones used by for the baltimore album quilts. I have great luck with it; if a newbie like me can make a stitch disappear it has to be good thread