Depending on who you are, where you are, and how you like to backpack and/or hike, everyone has a different list of "must haves" to bring with them. What would you bring for a day hike, an overnight, or a multiple day hike?
I have my core essentials, essentials for overnight, and essentials for cooking (if we group cook, my cooking essentials go out the window).
The bare minimum, core essentials that I currently keep in an Osprey Talon 22 in my car is:
Adventure Medical Kits Ultralight/Watertight .7 Medical Kit
A water bottle (drawn from an assortment of brands and styles) that I usually have with me, plus an emergency backup consisting of a Sawyer Mini & collapsable water bottle.
Morakniv Eldris knife on paracord lanyard w/magnesium firestarter (I recently threw a Lanksy Quadsharp in the bag for sharpening, but I might go back to a basic whetstone)
A couple granola bars
UCO Survival Matches
Lightweight columbia fleece I've had for ages
Hand sanitizer & kiss my face sunscreen
Outdoor Research Radar pocket hat
I also have a northface raincoat and a Tilley Airflo hat in my car that I'll take, depending on the weather. Likewise sunglasses that live in the car.
If I know I'm going hiking when I leave home, I'll add an appropriately filled hydraflask 2L water bladder & maybe a map of the area.
For overnight/multiday backpacking, my additional core gear is:
An Osprey pack I've used long enough I've forgotten the model and size
A Hennesey Hammock Jungle Explorer I recently started using with it's sundry accessories. I'm about to replace my mummy bag with a quilt, once I decide which.
A water filter (the sawyer mini is my backup). Currently either a Sawyer Squeeze or platypus GravityWorks, depending on length and companions.
If wood processessing is expected, I'll carry a hatchet, but I've recently replaced that with a SOG Kukri (but I haven't yet gone on a trip anticipating its use, and thus not taken it, so more testing is required).
A Petzl headlamp
An independent carrying method for my ten essentials (as I don't bring the Talon). My last few trips, this has been a Revamp Gear front pack vest, which basically adds detachable pockets to the front of my pack and which I can take off and wear seperately (I use this for my 10 essentials so I can keep them with me when I don't have my full pack on). So far it has worked well, but I'm still on the lookout for other options.
If cooking for myself is a component of the excursion, my core kitchen gear is:
Platypus GravityWorks Filter
Snow Peak Titanium Trek 700 Mug (I wrapped the handles in some kind of non-conducting self-adhering tape I got from the HVAC section of the hardware store a long time ago. I use the mug to boil water for cooking, cleaning and for hot drinks; then just add silicone hotlips if I intend to drink from it)
GSI Crossover Kitchen kit (replaced the included towel with the GSI Camp Dish Cloth and added a small pepper mill, a little spice container with salt, jerk spice & cumin, and small bottles of olive oil and sriracha, because why not - and it all fits)