One of the best is Tahki's Donegal Tweed. It has been a high quality yarn in the best of classic knitting tradition. It works up at 4-5 to the inch, is 100% wool with those wonderful tweedy flecks and wears like iron. Read a recent review that really sings its praises: http://www.knittersreview.com/article_yarn.asp?article=/review/product/120329_a.asp
If you are contemplating a warm sweater project to sink your teeth into, consider the Donegal Tweed. We have sweater quanities in a nice assortment of colors.