I've spent the last few months testing and reviewing over 20 sets of the best bike lights. After going through loads of good (and some not so good) rear lights, I rate the Magicshine SEEMEE300 as the best of a lot. It received an almost perfect score in my review, and the only thing I could find to mark it was that its mode selection was sometimes a bit confusing. 

It has the best battery life of any on the market at 200hrs, it is crazy bright, it auto darkens according to ambient lighting conditions In addition, it has unique features in the form of a down-facing LED that illuminates the large cone below you and greatly enhances the visibility of the sides. It sounds like a gimmick, but it's really effective.

A lot of useful for you, I also found it on Amazon with a healthy discount. It was already 10/10 for value at full price but could be also the best budget bike light on the market, with a 30% discount in the UK and a 25% discount in the US

US: Quick Cyber Monday Deals

UK: Quick Cyber Monday Deals

Here's how it works: Here's what I said about it when it was a great price:

"What's angry is how cheap this light is. It is packed with features, ranging from battery life, smart dimming, brake lighting, to downward light, that you would expect to pay the top dollar for it. In fact, it only costs £50/ま60 and is discounted very often. This is half the cost of the Exposure Boost-R and it's really a very good rear light, but it's not 2x. The value here is sensational."