Definition

Ravel cord should always be a hard finished smooth yarn such as silk, crochet cotton, or dress yarns.

While silk cord is expensive it can be reused indefinitely and slides out of the stitches quite easily.
Crochet cotton is cheap, readily available in many colors, and slides out of the stitches nearly as easily
Avoid fuzzy ravel cords as they can Ieave lint in the knitting and are more likely to break as the ravel cord is pulled
Choose a color that is markedly different from both the waste yarn and the main yarn.