The Ten Essentials
The prudent hiker carries the ten essentials at all times: even in a day pack. Judiciously applied, these allow a person to survive in an emergency whilst awaiting rescue.
Flashlight, because even if one does not plan on hiking in the dark, plans have been known to change.
Map and compass to find one's way. Given a choice, just a map is more useful than just a compass. It doesn't really do a lot of good to know where north is if one doesn't know where the car is.
Whistle and mirror will help searchers find the lost hiker more rapidly.
Extra food and water and extra clothes make the wait more enjoyable. A person can survive 30 days without food, three days without water, and three hours without warmth.
A knife is handy for a lot of survival tasks, like making kindling.
First aid kit for injuries.
Sunglasses, hat, sunblock to avoid snow blindness, sunburn.
Plastic tarp will protect against heat loss due to wind and moisture.
Matches and firestarter. Having actually spent a very cold night huddled around a survival fire at 9,000 feet in 40 feet of snow, I can attest that extra clothing is a lot more useful than a fire. If nothing else, every hour or so SOMEONE has to stumble through the snow to find more wet wood. But a fire can attract searchers and provide a psychological boost.