You have lots of choices, so the question is how much do you want to spend? On the inexpensive end, it's very hard to beat Performance's Viewpoint EVO, a ten-watt system that gives you adequate light for road riding and about two hours of battery life. It's a real bargainnormally $130, Performance is currently selling it for half that (www.performancebike.com).
The next step up is something like NiteRider's Digital Evolution ($229; www.niterider.com). I used a light like this for years for fast nighttime club rides, and it's great. It features a bright 15-watt bulb as well as six- and ten-watt settings to reduce light output and extend battery life (up to four hours on the six-watt setting). Moreover, a "fuel gauge" lets you know how much battery juice you have left. This is a great lightvery reliable and rugged.
Lastly, you can spend some serious scratch and get a true light cannon. Light & Motion and NiteRider both make what are called HID (high-intensity discharge) lights that use sophisticated circuitry, bulbs, and reflectors to cast a beam that's almost the equal of car headlights in terms of brightness. For two winters, I've been nighttime single-tracking with a helmet-mounted Light & Motion ARC light. It puts out an astonishing amount of lightbrighter than four of the 15-watt bulbs. Yet battery life is three hours. Of course, you pay for this sort of performance: $409 (www.bikelights.com). But to turn night into day, this one's the ticket.