My vote goes for the SON hub dynamo with a Lumotech Sensor Oval Plus. I cant detect any extra rolling resistance- it switches on automatically when it gets dark and does not go off when the bike is stationary. It is reasonably bright and there are no running costs.
I combine this with a very bright helmet mounted flashing LED light.
__________________
only the dead have seen the end of mass motorized stupidity
Plato
(well if he was alive today he would have written it)