A big flashlight, nothing special
LEDs differ from incandescent bulbs such as used in legacy flashlights in that you can't just connect a voltage source to a LED and expect something good to happen.  LEDs want to be driven with current.  Batteries are generally voltage sources and usually don't come in convenient voltages to run LEDs directly anyway.  Incandescent bulbs could be made for specific voltages over a wide range by changing the fillament length and diameter.  However, the voltage drop of a LED is dictated by semiconductor physics and isn't something the designer has much control over.

Fortunately, with modern electronics it is relatively easy to make a constant current source from a battery.  I have done exactly that for a head mounted LED light.  Follow the link and you will not only see a description of the project, but links to the schematic, board drawing, and BOM.