Depends on whether you are doing night riding or just riding at night..... Urban riding is completely different than commuting or dedicated night riding on trails or secluded country roads. I do mostly trail riding with a cross bike so there are roads involved too. I prefer at least two tail lights - PB Superflash or similar, one blinking, one steady. On the front it's a bar light with at least 500 lumens (in my case a Baja Designs Stykr at 700 lumens) and a torch of near equal intensity for the bar or on the helmet if there's any twisty trails involved. I prefer the bar to be a flood and the torch to be a thrower. This setup would also work very well for commuting.

A bar light is definitely the largest investment if you go with a quality light. For throw/backup, 18650 Torches are becoming ridiculously bright and have respectable run times. I have three - a P7, A C8 and an XM-L T6. Each could be used as a light by itself and would do a respectable job lighting the way home. At $23 to $30 each direct from China, they are the best lighting value out there. And you just can't find these types of lights at retail or from any U.S. sources that I'm aware of.

If you are doing urban riding with lots of city lights, the torches mentioned above or a self contained bar light is probably the way to go. Nightrider, Catey, Serfas, Lezyne all have nice (if pricey) options. The advantage of torches though is they can be used off the bike and have multiple light settings, plus you can carry spare batteries around with you. People's eyes are different in terms of night vision, but I would recommend 300 lumens as the minimimum. Again rear visibility is probably more important in this type of riding, so get one really bright taill light or go with multiples of the SuperFlash clones or similar.