As per CK... Torch of your choice. I helmet mount mine with an old Joystick mount (needed a few wraps of electric tape to bulk it out), works enormously better than the best Joystick tbh (though a bit bulkier).
I use an XML torch because it's for road use, but for offroad I guess I'd go back to my old P7 torch instead. Total spend on either was about £20 including spare batteries a cheap nasty charger- doing it again, I'd spend a bit more on the charger.
(I rant a bit about this, but it's not so much that the Joystick is just a torch- though, it is. It's that it's a torch, with one of the best features of torches removed- no easily replaced battery. So you spend a lot extra for a downgrade)