At the risk of "You're just saying that because you sell them" replies... :-p
My Reevu helmet. With my panniers fitted to my bike all I can see in the mirrors are the panniers, so the mirror in the lid is invaluable for touring. I did 1500 miles around Spain in July, just using the lid's mirror and loved having no blind spots anywhere.
But even without that, it's still the quietest and the most comfy lid I've had in nearly 12 years of riding, better than my previous Shoei XR1000 and Arai Tour-X by a long way and considerably cheaper.