You don't post any information about how long you want the light to run.
This is just as important as how powerful it is. It can change the price a lot or a little. Narrow this down first. If you will be riding in the cold you may have to buy more run time than you need in the warm weather.
Get a light with an automatic charger.
If it does not say this in the ad copy it probably does not have one. This means that If you do not have an automatic charger, you have to time your charge or you can damage your battery.. If the battery is half full you need to charge it half way. A major pain, no reason to get a light without an automatic charger. There are plenty that have them now.
If the run time is long enough, and it has an automatic charger get the most powerful light that your budget will allow. There is absolutely no such thing as too much light.
If you are torn between a couple of similar lights, maybe the battery type will help make the decision.
Li-ion best
NiMh very good
Nicad almost as good as NiMh
Lead acid fine battery but heavy and big.
Then ask around about quality or look for reviews.
This light has everything. But no well known brand name. I would risk my money on it.
http://www.batteryspace.com/index.as...OD&ProdID=1367