After 40+ years of tramping etc I still find it useful or at least reassuring to at least glance at a list before heading out the door.

There are plenty of (good) opinions out there, every overnight trip is slightly different, and with in each category (or line in this list) there is usually many choices to make ... but  everybody I know would probably tick at least once for most of the following lines ... on most trips ....  

food

pack / waterproof liner(s) 

walking clothes

boots / gaiters / walking shoes

sun hat / sunglasses

rain jacket / overtrousers

warm hat / gloves

light spare clothes (for evening/sleeping)

sleeping bag

ground sheet / mattress / cover /  fly / tent

pot

cooker / fuel

lighter

bowl / mug / plate

spoon

head torch / spare batteries

pocket knife

map, compass

...PLB (at least one per group is fast becoming the norm).

tooth brush / paste / soap / towel

toilet paper

first aid / spares and repairs