My tuppence - ALL THE LIGHTS!
A handlebar lamp is what vehicle drivers "expect" to represent your forward direction. So I have a high-power chinese thing with 4x18650 in a pouch. I only take it in winter or when I expect to be riding in the dark.
I'm not sure if it works better on the bar or on the head-tube... with the headtube the light doesn't wiggle as much, but it doesn't light up around corners so well
I also use a blinking pinpoint torch on the helmet which I use to light up reflectors and point out to drivers where I am. Stays on upper rear of helmet all the time
In addition I have blinkies on the front and rear, because winking gets attention at the sides of the driver's vision as opposed to direct-on. Again, always on bike.
Multiple lights works well for me, because I've had the occasional one go flat while riding. If its a rear, then you won't know till you get to your destination.
A super-bright headlight has caused me problems on a night ride - it was dusty, and the head light illuminated the dust right in front of my face, obscuring vision and making me go slower. Another reason for putting the big light on the handlebar.
Some locations have restrictions about what you can have blinking and what must be on solid. Bear that in mind, and do what keeps you safest.