I use a light one from Joanns, but can't remember the name. There is a wash-a-way that you can get from machine embroidery, but I think it is pretty expensive.
Tip: I just tried this last time I appliqued. Draw the pattern on the fusible backing like always then cut out the center - leaving about 1/2" of border then iron this onto the fabric. It cuts down on the weight and then you can use that cut away part for more appliques. It did take more time in preparing them, but I liked the feel much better. Not so flat and stiff.