The main purpose of a bike tail light is to make yourself visible to cars. You make think this only applies to nighttime, but many people have noticed a definite improvement in the amount of space cars give them when they turn on their bike tail light in the daytime too.
For this reason, it is really, really important to get a bright light. The best way to find a good bike tail light is to read product reviews.
For instance, the Planet Bike Blinky Superflash has 183 five-star reviews. One review said, " This light may actually be too bright for nighttime use, that’s how bright it is."
Other really bright bicycle taillights are ones made by DiNotte. They’re like flood-light bright. But the DiNotte costs about $130 compared to the Planet Bike which is under $25.
Here’s a link to Amazon which has the Planet Bike tail light:
Planet Bike Tail Light (affiliate link; I get a few coins as a thank you for referring you).
Planet Bike Tail Light (non-affiliate link).
Another important feature is to have a flashing option. Just the way people tend to ignore sustained sounds, even if they are loud, people may also ignore a light, even if it is bright.
But a blinking light tends to get noticed better. It could be because it makes us think of something needing attention, like when a light is flashing on the car dashboard. Or it could make us take notice as if it were a police light.
One reviewer said there was a noticeable difference when he put his light in flash mode during the day. Cars stopped passing him in the right lane and gave him more space.
Something else to look for is a decent casing because you don’t want to spend a lot of money for a really good bike tail light, only to have the casing break in the first month when you remove it from your bicycle.
Also make sure that the bike tail light can be seen from the side a little instead of just dead-on. That way you’ll still be seen by cars that are changing lanes into yours, not just those that are coming up to you from the same lane.