I find that, because lighting technology and models change so frequently, it's hard to recommend specific lights. I usually visit MTBR.com or another mountain biking site to get the latest info and reviews. Those guys know lights! See this year's "shoot-out": http://reviews.mtbr.com/2012-bike-lights-shootout

In general, the more light you have, the faster you can go. My winter/pitch black light is a Nite Rider Tri Newt from a few years ago. It's ~500 lumens, and I wouldn't want any less light. However, the prices on LEDs have come way down, and I could probably buy a 800 or 1000 lumen light today for what I paid then.

I have had reason to use Nite Rider's customer and repair services, and the whole experience was quite painless (other than not having my light for a couple of weeks). Based on that alone, I'll consider another Nite Rider when this one gives up the ghost.