A flashlight or headlamp by itself is sufficient, although many hikers or backpackers carry both.
Carrying both provides a second source of light as a backup, in the event one source is lost or damaged.
A headlamp is useful since it keeps the hands free, and may be comfortably worn if hiking must be done at night.
LED flashlights or headlamps are better than conventional-bulb lights for continuous outdoor use. LED bulbs are brighter and greatly extend battery life when compared to regular bulbs. LED bulbs are also much more durable when dropped or used roughly.
Carry a spare bulb if your light is of the “conventional” (non-LED) variety, and always have a fresh set of batteries in any light at the beginning of the trip.
Carry a replacement set of batteries dedicated to your light(s), LED or non-LED. This is a spare set, in addition to the fresh batteries (see above) already in your light(s).
Know how long batteries last, and gauge use accordingly. Knowing how long a light will remain functional is essential in an emergency.