Coats "extra strong" Dual Duty Plus Hand Quilting (glace finish cotton covered polyester) and A&E Quilting Thread (glace finish cotton covered polyester) are probably the strongest threads I've used but my two favorites are Robinson Anton 100% cotton 50wt and Mettler Quilting 100% cotton 40wt. They tend to tangle a bit easier than the glace threads, but they make a pretty stitch and come in so many wonderful colors.