There are lots and lots of tail lights out there. My main advice here is not so much about which as about how many. I'd have two, or more. Two lights make you twice as visible (at least). One could argue that two provide an element of depth for following drivers. And certainly, if one fails (batteries, malfunction, whatever the cause), you're so much better off with one than none.
Additionally, make certain they can be seen. No obstructions. Able to be viewed from the sides.
Now for why I like this light we get into intricacies. I like the "Senso" feature that turns it on when the bicycle is moving or if you go through a tunnel and it is suddenly dark. How often do you go through tunnels? Not often. But when randonneuring you likely don't know whether you will or won't. But have you ever ridden through a dark tunnel without a tail light. It feels very vulnerable. That's exactly when you do need a tail light.
And if you've paid attention to any of the stories about riders hit from behind by motorists--those who have been killed and those who haven't--it will wake you up. There just isn't too much when it comes to lighting--especially tail lighting. For that reason I don't now own, but have been considering, helmet rear lighting like the photo below.
Problem with these is that they flash, which would be very annoying to those cycling behind me. However, when night riding alone, which I did for a long while on that first ever through-the-night brevet, I think it might be smart since there isn't the additional movement and lighting of riding partners.
Below is a picture of the tail light I got for DartreDame's bicycle. Because she has a rear rack it mounts very easily, is mounted very visibly right off the back, and it incorporates reflectors with the light. Nice and big. I like that for her. I'd like it for me, but not enough to install a rear rack. Hers also incorporates the "Senso" feature.
In case you still need some motivation, do check out these accounts of motorists hitting cyclists from the rear:
For more on Headlights, see Lighting for Randonneuring, Newbie Style, Part 1: Headlights.
Keep it lit up,