Knog Blinder V4 on the helmet, mounted sideways, very bright and noticeable.

However, there is a problem with this set-up. I wear a backpack, and when I'm in the drops or otherwise pretty low, I fear that the backpack may block drivers' view of the helmet taillight.

I have rationalized the situation by figuring that if I'm in the drops, I should be going fairly fast, but that sounds stupid even as I type it. Well, I have a blinky on the bike as back-up, and do monitor my helmet mirror constantly.