What I use depends on the look I want for the quilt -- the quilt usually tells me what it wants. I have used cotton (I like that for table runners and placemats since I may put hot things on them), I use shiny poly for a lot of FMQ but I also use metallic thread when I want a special look. I have couched thicker threads and braids that do not go through my needles if I want something to stand out (pearl cotton, funky yarns and metallic). I really hate to limit myself to one type of thread since there are so many fun threads out there.