Planet Bike Super Flash.

Not the cheapest, or most expensive. But I think the best bang for the buck. I'm very pleased with the performance of mine, as are many others on the forum. It's been my experience that PB backs up their products really well.

http://ecom1.planetbike.com/3034.html

It does not come with a rear rack mount, but you can buy one from them for $5, free s+h, on their website. (link below)

http://ecom1.planetbike.com/3108.html