Hello and welcome to the board. I do a lot of raw-edge appliqué and as the others have said, it's up to you what you use and what effect you're trying to achieve.
Some like their appliqué to blend in, so use a cotton that exactly matches their piece. Others like a little emphasis on their hard work so use a rayon or polyester.
There are whole articles about what thread to use - some will tell you cotton, some silk and some will say rayon.... Others believe that a trilobal polyester is better than rayon if you like a sheen on your work.
Possibly not the definitive answer you were hoping for, but happy appliquéing!
