Best is clearly a relative and subjective term.
For me, reliability is my top concern.
If the light fails after being dropped, or falling off the bike its worthless.
Predictable battery life is also just as important, runtime matters. (5 hours minimum has been sufficient for my needs)
For me, I consider my rear light (in the I picture linked to in a previous post) bright enough (in flashing mode) for urban use, it lights up street signs 200 meters away.
I would recommend something significantly brighter if the roads you ride on have massive speed differentials, also be aware that overly bright lights may cause target fixation for some road users.
Last edited by SHBR; 01-21-18 at 09:51 PM.