Hi Ricdot,
Where to start?
Toothbrush, clean socks, pillow, camera, knife and fork, only kidding

It is a tricky question to answer as I think it mainly depends on where you plan to go, what you plan to do and for how long.
We packed everything imaginable when we travelled for 5 years and found for months on end something wasn't getting used. Examples would be heavy winter clothing, fishing gear and books. So a lesson learnt for us was to have the basics then buy what you need as you go if on a very long adventure. I soon found many book exchanges to cut down on book storage, Smed hired scuba gear when the fishing scene changed and an allrounder flanny and jacket sufficed as opposed to a full winter wardrobe.
This may answer 3% of your question but something to think about.
Cheers,
Sharon