In my experience as a driver (and a biker), the Busch and Muller Toplight Line Plus is the best dynamo tail light:
Busch and Muller Toplight LIne Plus

For some reason they don't make any dynamo tail lights that flash (well the reason is that they don't allow flashing lights in the countries dynamo lights are primarily made for). Obviously any tail light is better than none, but the Toplight Plus is the only one that as a driver I feel comes close to standing out and identifying a bike as a bike compared to the flashing versions.