Thick and soft cotton socks. They fit nice and snug, and won't slip off of little kid's foot inside a boot.  The last point was important to me, because with other socks we'd have to stop so many times to take the boots off, and pull up those socks! Not these. Once on the feet, that's where they stay. My son loves them too. They keep his feet warm and toasty. Non-skid is great too, since he doesn't seem to know how to walk, and we have stairs and wood floors.
All these little things make me happy :)