I haven’t read the book myself, but I’ve heard that “World War Z” by Max Brooks deals with this topic alot. Not so much in the movie - obviously.

I honestly don’t know which would be the ideal form of governance but I think I slightly lean towards groups choosing a leader which would then effectively become a dictator of sort. Ideally there would be many groups though so if you don’t agree with the actions of one, you could join another. I have a feeling that democracy might not be the best choice especially initially. Life in a post apocalyptic world mandates that very difficult decisions will have to be made.

Scavengers form groups. Groups become run by Warlords. Warlords absorb or destroy nearby groups. Warlord territory spreads to be regional. Regional powers give rise to proto-countries.