The kevlar shell Gentex HGU-55 is probably the lightest of the lot - the David Clark shells which go over a DC headset are light enough in themselves, but the headset adds quite a lot of weight. Not to mention the, ah, appearance. The UK Alpha helmet in kevlar is also quite light, but comparatively bulky, not to mention expensive to source spares for, whereas the Gentex spares are cheap and easy to come by - but the Alpha I'd argue offers the best protection by a small margin.