I have two of the Cygolite HotShot Pro tail lights (one on each bike). For my bike that has a rear rack, I purchased a rack mount for it on Amazon for just a few dollars.
The light is so bright that I get comments from drivers and even from the police (one time an officer pulled a u-turn so he could come ask me what light I was using, so that he could recommend it in his bike safety program). It's also small, and USB rechargeable. And it lasts a good long time on a charge. It can be mounted on the seat post, which is where I usually mount it, but when I have something on the rack that obscures the seat post, I move it to the rack's adapter mount.
If I needed another light for another bike, I would get the same one again.
The rear rack mount is easy to use; it connects to the plate at the back of many of the better racks on the market (such as the Tubus series). And then the light just snaps in.