Going back to post number 107 on the forum, you may find
this topic has all the answers.
These days I use a spreadsheet with tick boxes down the side. So I check off the items I think I will need on each walk, whether it's a day walk or an overnighter.
Bear in mind I walk with my wife so some items here may not be relevant...
Picking a single must-have item, hard to do, but you are welcome to peruse my full list and see if there's anything that grabs your attention.
Note, the wine bladder isn't on the list, and I have a separate list for food. Licence, credit card, and a little cash normally get put in as well - mainly for the trip home.
GearPack
Pack liner bag
Garbage bag for clothes
Garbage bags for sleeping bags
Sleeping bags with silk sheets
Sleeping Mat
Tent / Poles / Pegs / Snowpegs
Tyvek Groundsheet / Space Blanket
GPS
Torch
Spare Batteries for torch & GPS
Map/s
Compass & whistle
Lip balm
Insect repellent
Sunscreen
Toothbrush
Mouth Fresh drops
Toilet paper
Trowel and tent peg for digging
Plastic bags for rubbish etc
Water bottles
Water bladder
Pocket knife
First aid kit
Stove with lighter, matches, etc
Fuel
Billies
Bowl / Cup / Spoon
Tea towel in bag
Female personal items & medication
Notebook and pen / pencil
Scourer
Sunglasses
Spare cord
Guy rope for tarpaulin / tent rope
Moisturiser
Thermal accident blankets - St Johns
ClothesBoots (snow sealed)
Gaiters
Japara
Plastic pants
Socks, minimum 3 pairs
Undies
Thermal pants
Thermal top/s
Shirt/s
Trousers
Jacket / jumper / windstopper
Vest
Hat
Beanie / balaclava
Gloves
Clothes compression bag
OtherCamera Gear
Food
Phone
Parks pass