Rule the first: The crappy flashlight you have on you is better than the awesome flashlight you don't.
Okay, I lied: Have a flashlight on you whenever you can. Emergencies don't always happen in the comfort of your own home. Tiny keychain flashlights may not be very bright and they might run on hard-to-find coin cell batteries that you need a jeweler's screwdriver to change, but having a crappy light on your keychain can help you get to a better light (like the one in the trunk of your car or your desk drawer.) And in an emergency, any light is better than no light.
Even if it moos:
Non-mooing coin cell keychain lights:
Corollary to the first rule: Your cellphone doesn't count as a light.
But, you might say, I already have a light: my cellphone screen lights up -- I even have a iPhone app that turns my phone into a flashlight! And you just said "any light is better than no light"!
Other than the ridiculousness of using a $500 phone to imitate a $0.99 flashlight (and poorly at that), save your phone so you can use it as a phone. Not only do you not want to run down your battery in case you need it to actually communicate during an emergency, but the more you wave your phone around trying to use it like a flashlight, the more likely you are to drop it, which means you're out both a phone and a light.
Rule the second: Brands don't really matter, but batteries do.
As a flashlight aficionado/geek, this one actually pains me a bit to say, but unless you're in a public safety profession where you need a fancy specialty light (weaponlight, hazardous environment rated, dive light, something to cut through smoke or forest, etc), pretty much any modern light that you can get these days at CVS, Sears, Target, Home Depot or Wal-Mart will serve you in an emergency.
Yes, using a modern flashlight with LEDs (no bulbs to worry about burning out or breaking), power regulating circuitry and ruggedized/waterproof casing is helpful. But (and this is important), as long as it uses easy-to-find batteries (no watch or camera batteries), you'll probably be okay.
In an emergency situation, easy-to-find basically means AA and AAA batteries. For us oldsters who didn't grow up with eBay and DealExtreme, C and D batteries are still ingrained as "flashlight batteries," which means they disappear from stores pretty fast. From anecdotes as recently as the Northeast blackout of 2003, C and D batteries were cleaned out, but AA and AAA batteries were available.
Rule the third: Go hands-free
A good quality LED headlamp can do pretty much anything a handheld can do, at least for what a typical person would need in an emergency.
Headlamps tend to have wider flood and lesser throw, but you can hold them in your hand (wrap the headstrap around your wrist) as well as wear them on your head (duh) to keep your hands free when you're making a sandwich or changing a tire. (Just try not to shine them into the eyes of the people you're talking to, it's annoying.)
Rule the fourth: In emergencies, output is okay, but runtime rules
In an emergency that leads to an extended power outage, unless you're in one of those aforementioned specialty occupations, most people will find that runtime is far, far more important than brightness. When it's truly dark out, a light can become "too bright" to be usable pretty quickly.
LED, cold fluorescent, or even electroluminescent lanterns are plenty fine for typical indoor use -- adjustable settings will also let you find the right balance between usability and battery life.
Rule the fifth: More is usually better
You don't have to be like me (a mostly-reformed, formerly unhinged flashlight collector -- but you should see those other guys), but you should have a couple of lights, so you don't have to cannibalize your emergency kit for a light for the kids on Halloween.
Preferably, have a light in each one of your emergency kits, one in the glovebox of each of your cars, and a few others in the house (master bedroom, kitchen drawer, utility room -- preferably enough so you have one for each member of the family so the kids don't fight.)
No matter how many flashlights you have, check them periodically to make sure they still work, especially if you store them loaded with alkaline batteries, which can leak acid and cause flashlights (especially modern LED ones) to break. Lithium batteries are more expensive but don't have this problem.