Just wanted to add an opinion. I'm not going to favor one particular rear light over another as there are so many good rear lights nowadays that it just wouldn't make sense. I think if you are using a rear light that is rated to be over 100 lumen ( usually this is in one of the flash modes ) that you have something that works and will definitely get you seen at night and perhaps even in the day. Quality though is important so I would definitely recommend a good brand name light.
Example of a good light: Some weeks ago I was driving my car in long line of slow moving traffic. Line of sight was very limited and most of my attention was aimed at what was directly in front of me. This was during the day but somewhere just before sunset. Suddenly my attention was directed to my right by a very bright light. Indeed, I saw the light first. Cyclist using a very good rear light in a place and time of year I would not expect to see a cyclist ( must of been a fairly warm day ). I was so impressed by this that as I came along side the rider I wound my side window down and told the guy that I really liked his rear light. Not sure what he was using but when I caught up to him again the light appeared to have the configuration of a
Cygolite Hypershot ( two LED's, two optics, one of top of the other ). Possibly there are other brands that use the same physical configuration so I don't know really what he had. Doesn't matter. What he had worked and worked well.