Happy with my Cygolite Expilion 850. USB rechargeable, water-resistant, multiple levels of brightness, can be mounted on helmet or bike, etc.

The MUP I ride on for commuting can get pitch dark at night. My colleague recommended a helmet light to make it easier to spot flat-causing debris and other hazards in the dark. Yet another nice thing about the Cygolite is the included helmet mount includes an angle adjustment, so you can set it to illuminate the ground in front of you instead of blinding motorists who might be driving in the opposite direction.