Our favourite XC helmet for a couple of years, Met's Fifth Element is an airflow lover's dream lid.
Massive vents and deep internal channels are separated by deep spines, so from the front it looks like there's actually more helmet missing than there is left. This translates to masses of airflow over your noggin too, making this the coolest helmet around, even at relatively low climbing speeds.
The adjustable antennae attachment of the thin peak means you can block out more weather, headlight glare and sun than you'd expect, and it's securely anchored, too.
Overall fit is also good, with skinny removable pads that don't interrupt airflow and a robust, widely adjustable headlock at the back. The strap is easy to adjust too, and the hard shell protection on the outer skin keeps it good looking for years without spoiling the impressively low weight.