The ideal pillow keeps your neck aligned with your spine while you sleep — not too high (tilts the neck forward) and not too low (lets it drop back). Back sleepers usually do best with medium height, while side sleepers typically need a higher pillow to fill the shoulder gap. Sleeping on your stomach is the most demanding position for the neck, since it keeps the head turned to one side for hours. Just as important as material is shape retention — a flattened, worn-out pillow loses support no matter how good the original material was. There's no single 'best' pillow; testing based on your own sleep position is the most reliable approach.