I vote for the Cygolite Mintycross 350,
I have been using it for about two weeks so far...
It comes with a bike and helmet mount.
Is very small. Uses a rechargable LiIon battery (charger included)
Has a long operating life.
Is weatherproof (I doubt any light is waterproof, submersible)
And is plenty bright enough for even unlit trails at reasonable speeds.
Paid $175 from amazon with free shipping.