Okay, for power failure, the top two most useful lights are probably not the kind of lights you're thinking about. The top two most important lights, IMO, are:
1. A headlamp. You'll still want to be able to do things like cook a meal on your gas stove, and you want two hands for that. You'll want two hands to search through your cabinets. You might want to do some reading. Plain and simple, nothing beats a headlamp. The other things you want are long life, and a choice of brightnesses. You can get a Princeton Tec Aurora for just over $20 -- at the price, you can buy one for each of you. It has three levels of brightness, it's on a hinge so you can point it wherever you want. Go buy one, and I guarantee you'll use it more than any other light people recommend, and you'll thank me for it.
2. The other light that can come in handy is some kind of fluorescent lantern. Sometimes it's nice not to have to sit in a dark room for days, and just running it a half hour here and there during mealtimes makes things more bearable. I don't have a specific recommendation for lanterns, but they're easy to find.
Those two are, IMO, the most useful kinds of lights in a power outage. But if what you really want is a handheld light, I feel you're looking for the same attributes I mentioned in the headlamp discussion: long life and dimmable. My eternalight x-ray plays this role, and I sometimes stand it on end and use it in candle mode. If you want something brighter as a just-in-case light, get a UKE 2L. Lithium batteries can go years without use and still run fine, and the UKE is a great tradeoff between runtime and brightness. The elektrolumens blaster that was described above sounds like it has nice characteristics too.
Joe