Planet Bike Greatful Red.
Besides the coolest name ever they are only $20 and oversized so it counts as a passive reflector when not turned on.
You WANT the AA battery version. Load it with eneloop rechargeable batteries. I go months between battery swaps. The eneloop batteries are low self discharge, meaning they will hold a charge in a drawer or unused on your bike for a year. Buy a stack of these and put them in every remote or clock in your house than keep extras in a drawer. Recharge time is the 30 seconds it takes to swap batteries. If on tour can swap in plain alkaline if needed.