This is the first time I've designed a cowl! Sienna was inspired by the gorgeous book Sequence Knitting paired with the luxury blend Epiphany by Cascade. Unfortunately this is a discontinued yarn but as always I put other yarns that will work in the Ravelry suggestions.
Sienna is a simple cowl with a two row repeat that is so easy to memorize. As stated above I used a luxury blend but my testers used sturdier wool and came up with beautiful cowls as well. If you want a cowl with drape then having some amount of alpaca, silk, or bamboo would be an excellent addition to your fiber content.
If you are looking for something a bit sturdier that will have more structure then I would use a merino wool or a wool nylon. Acrylic or cotton would work for Sienna as well but make sure to use a corresponding needle size so the cowl doesn't end up too stiff.