Tip:
Flashlights are very important for identifying a potential threat, and often can be carried in places that you may not be able to carry a firearm. Flashlights are a great way to show that you are a hard target and that you will not be an easy target, while simultaneously being a benign tool that harms nobody if you make a mistake.
Handheld vs. Weapons Mounted:
Weapons mounted lights are very useful for the needs of shooting but is more difficult to safely use for identification unless you have training with the tool. Additionally, it may be difficult to carry a firearm with a light attached. The main advantage of a weapons mounted light is that you have both hands on the gun when firing.
Contrarily, the hand held flashlight can be used independently of the firearm, and is more useful for everyday tasks. It can be used with a firearm, but may be more difficult to become proficient with. It also eliminates the potential risk of an assault charge if you use it to identify a potential threat.
How bright:
There is a constant debate on how many lumens is enough or too much. The problem with the "Too much" argument is that what seems capable may not be enough for the next situation. Your flashlight should be capable of cutting through darkness both inside and outside, with a target that is backlit, and if the target has a flashlight in your eyes. Perhaps most importantly, you need to be able to identify your target and WHAT THEIR HANDS ARE DOING. To accomplish all of these tasks, it may be advisable to find the flashlight with the highest output available. Surefire and Streamlight are the most highly regarded options on the market, but companies such as Elzetta and Blackhawk have good reputations as well.