Kirkland (ie Costco) Merino blend women's socks. Sooooo warm, sooooo comfy. Big enough for my size 7.5 hooves. Thick and cosy, yet still fit comfortably in ordinary shoes. Only drawback is that the elastic top is a bit tight for my big calves, but the sock tops are long enough that I can fold the elastic down to my marginally narrower ankles, and still get good coverage.
Haven't washed them yet, but am utterly besotted with them.