No one person in the world is capable of making a pencil from scratch! [1]

Given that, i would think the big factor in building anything post-apocalypse is communication. So i guess my answer would be paper and pens. Hard paper copies of as much information as possible and blank paper to write letters.

Computers are the end product in a long long tech tree. So the question just seems sort of non-sensical to me. Like if we get to the point where we rebuild society enough to even consider computers, we have already solved food, shelter, heating, manufacturing, mining etc. At that point its mostly a matter of information - just archiving as much info as possible on stuff like silicon manufacturing, linux source code, electronics etc. would be enough.

[1] https://www.econlib.org/library/Essays/rdPncl.html?chapter_n...