What you need depends on lots of factors. Just don't be cheap.

Three good brands of battery lights are Light & Motion, Cygolite, and NiteRider. I've used a few Cygolite lights, and they are well made and reliable. Customer service is also very good.

But generally, I use a dynamo powered system. Yes, it's expensive, but I love the reliability. I get full brightness every minute of every hour the bike is rolling. The only time my headlight stopped working was when someone -- I presume -- saw my headlight shining after I had parked the bike and "helpfully" switched it off. I never touch the switch, so I didn't think to look at it until I got home. Dynamo lights have capacitors to keep the lights on for a few minutes, so the passerby must have thought I was burning a battery.

Then again, I just got this $10 light as an experiment, and I'm impressed. It puts out a shaped beam like the European lights, and it's brighter than my dynamo light. The run time is genuinely two hours as per my test. I don't trust it yet, as I haven't had it long, but it could work out well.