What is the feature of a helmet that provides the best protection?

A snug fit is crucial for a helmet's effectiveness in protecting the wearer. When a helmet fits snugly, it stays securely in place during a crash or impact, allowing it to absorb and distribute the force of the impact effectively. An ill-fitting helmet can shift, slide, or come off completely, significantly reducing the level of protection it offers.

In contrast, factors like a lightweight design may enhance comfort for long rides but do not directly influence the helmet's ability to protect during an accident. Similarly, a colorful appearance might be aesthetically pleasing but has no bearing on safety. Lastly, while a ventilation system is important for comfort, especially in hot weather, it does not impact the protective capabilities of the helmet. Thus, the snug fit is the feature that fundamentally ensures the helmet performs as intended in the event of a crash.