H53w with mount works incredibly well on the helmet. Very lightweight, quite bright for an AA light or as dim as you want and you can reposition the lamp as you ride.

Torch:
H53w AA Headlamp Neutral White

Holder
GITD silicone holder for H51, H502, H52, H53

I've upgraded to the 18650 version for the increased brightness and runtimes but the AA ones are more comfortable.