"Reliable" has a number of meanings. On the rear, I'll go with at least two AAA lights for redundancy -- Planet Bike superflash and a nice Cateye blinkie. Check nightly, and if one looks substantially dimmer, change the batteries. That way I'll usually have at least one working.

On the front, I like Cygolite and my LBS likes Light and Motion; so when the old battery gave up the ghost, I ordered one and bought the other so I could keep coming home from work.

Note the real problem is battery charging and reliability; anything with batteries will wear out, usually in half the time the specification says it will. So, like fietsbob, my commute bikes have dyno lights front and rear. They only give out when the rider does. I got the first one halfway through the winter I bought the two front lights, and I liked it so much I gought another wheel + light combination the next winter. (I still like the blinky + steady dyno light on the rear, and I've put the Cygolite on my helmet because my wife wants me to have something flashing, although I find it distracting.)