The most beautiful hijabs that I have ever seen are Anokhi's rectangular scarves which are large enough (15" by 72"), have a gorgeous range of patterns and are soft cotton voile. The scarves cost USD $14 and include the price of shipping to the US & Canada. They are made in India, block printed and embroidered by hand. You can order by colour groups and there are patterned and solid scarves.
Anokhi also sell square scarves of 30" but these are too small to properly cover the neck and chest as can be seen on the left, if you were to wear an al-amira underneath however they would fulfill the obligations of hijab as I have shown on the right.