According to a man who should know, flannel, soft woven wool with a distinctively mottled face, was invented in the 18th century to meet the requirements of continental European armies that were seeking a snag resistant uniform cloth. Three hundred years later, the stuff remains to my mind the best choice for winter suitings when it is woven from merino yarns or a mixture of wool and cashmere.
Flannel wears warm and has usually been woven to about 400 grams in weight, making it a cloth for freezing climates (there are newer, hybrid worsted versions of 310 grams or lighter that wear considerably cooler). The classic weaves are gray and navy solids, chalk stripes on gray or navy grounds, simple windowpane checks, black on white houndstooth, and of course the glen checks popularized by the Kings of England, in black and white, grays, or tans and browns.
Men who wear tailored clothing could do worse than to have a winter rotation of half a dozen flannel suits in their closets.