The Cateye Rapid X3 is the best taillight that I have tried.
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The battery life is excellent and I have one on each of my bikes for convenience. If a battery is dead at the beginning of the ride, I will just switch the light with another bike and set the dead one to bake on the charger. They charge up quickly, too.
Because I liked the Rapid X3 so much, I got a white one for my primary blinky up front. The Niterider Lumina mini 400 is my second front blinky.
That being said, I am a believer in diversification. I also run a blinking slapband ($2+/-) on my left ankle for the 2 or 3 miles up to the trails on the MTB and a second taillight on the roadbike 24x7.
This way Mrs. Gock's lawyer can ask the SUV driving soccer mom in the deposition after I die whether she missed all 16 blinking lights, when she inevitably hits me by accident.