You may not realize it, but one of the most popular apps on any phone is the flashlight app. Some phones, don't have one and you have to go looking in the Play Store for an app. The HTC One M9 doesn't actually have a flashlight app per say. HTC has added the flashlight toggle to the quick settings. So to access it all you have to do is pull down the notification tray and hit the Flashlight icon in the quick settings section. It's pretty simple and easy to use.
It's a great place to put the flashlight. And to keep the app out of the app drawer was a great idea by HTC, and great to keep things simple. Especially seeing as a lot of other devices have the flashlight app in the quick settings now, including my daily driver the Sony Xperia Z3.
Additionally, HTC has made the Flashlight app an option for the on-screen buttons on the HTC One M9. So if you use the flashlight app a whole lot, you can put it right next to the recents key or back button to make it even easier to use. You can also customize its spot in the Quick Settings or remove it altogether.
So in short, getting to the flashlight app in the HTC One M9 is as simple as pulling down the notification bar with two fingers and then hitting the flashlight icon and boom. You're all set.