From MORC Wiki
There are many types of bicycle lights on the market. The two basic uses are for commuting or off-road riding. In general a commuter light is not bright enough for off-road riding. While riding off-road it is important to see all the trail features. You need a powerful light when off in the woods with no outside lighting. A commuter light is mostly designed to allow others to see you, not so much to light your way. So choosing the right light is not as easy at things may seem.
Basically a 10 Watt equivalent light is the minimum light you would want for off-road riding. That being said, many new technologies have come around in the last few years and HID and LED lights are becoming more popular. Mostly because they give more light output per Watt of power than a traditional Halogen light. So the 10 Watt rule doesn't apply to some of these newer lights.
Many people ask whether it's best to have the light handlebar mounted or helmet mounted. In most riders opinions it's best to have the light mounted on your helmet. If the light is on your handlebar, you can't see around corners before you start to turn into them. Also, the light bounces up and down a lot with the bike. When mounted on the helmet, the light will shine where you look and be more stable, meaning less shaky. Some riders will have both, a handlebar mounted light and a helmet mounted one. This method works quite well, but you probably want to have two lights with a similar amount of power to equalize things.
There are many brands to choose from. Not everyone agrees on which ones are best. Here are some links to popular light brands.