Short answer: Take $350 to your LBS and be prepared to part with it.

Long answer: Every reputable bicycle light manufacturer has models with very bright, very wide beams for off-road use. I don't know which one's best for you, but I figure if you have a problem with something you buy from your LBS, they can handle warranty service (if needed). If you're going off-road at night, I suspect you'll want something designed and built to be rugged, waterproof, and to fit well on your bike. That last is why I'd go with a bike light, rather than the brightest flashlight you can find and bungee cords.

And take the Flea along, just in case you need to walk out of the woods.