Cashmere
Real warmth at very little weight, which is why cashmere belongs under a coat rather than over one. A short list today, and it will grow. See wool versus cashmere.
What to know before buying cashmere
Cashmere is warmer than wool for its weight and softer against the skin, which is why it works close to the neck and under a coat. It is also more delicate. Expect a little pilling in the first weeks of wear; it is removed with a cashmere comb and slows down after that.
Wash it rarely, by hand, in cool water with a wool detergent, and dry it flat. Fold it rather than hanging it.
Compare the two fibres in wool versus cashmere.