First, think about power type. It's unlikely you'll have access to electricity so USB flashlights aren't typically a great option (unless they have a backup power source). Generally speaking, flashlights that run on either solar energy or regular AAA batteries are best. If you can find one that offers multiple power types, that's even better.
Second, consider battery life. If your flashlight runs on a rechargeable battery, make sure it can hold power for at least eight hours in an emergency — preferably longer. If it uses regular AA or AAA batteries, buy high-quality, long-lasting batteries and keep extras in your emergency kit.