I've just bought this Lezyne e-bike light which I havent fitted yet, its really small and superbly built and with 1000lum output should be pretty bright! It comes with bare wire terminals for running down for direct connection to the motor power outputs but I'm planning to feed through the down tube for tidiness, fingers crossed.
I would get something that connects directly to your motor. It can be turned on or off over the control menu (Though this is a royal pain in the bum on the Shimano system). On Bosch it is easy (once you have had the function activated).
I used the IQ-X from Busch and Müller. 150 Lumens and great quality.
It is middle of the price range. Have had it on my bike for nearly 3 years now without problems