The first company that came to mind is Unity. Epic has Unreal and actively dogfoods it by developing Fortnite. But Unity doesn’t really have a flagship product like that, right? I'm sure they use it internally to build sample games, but nothing at AAA scale?

I think there are probably some out there, likely would have started out as a niche contract job (like say fast food POS systems) and have built up from there. Though its really hard to make something like that "great" unless you hire experts from that field to help develop them.

Well software for non-software people...

Like I doubt that people writing autoCAD are ones who needed it in the first place.

Maybe I am wrong.

The US Strategic Automated Command and Control System (DDSACCS), which sends emergency action messages to nuclear forces.

Not entirely software .. but an example of a system not used in house .. or at all ... yet.

Greatest? Hmm. Hard to say.

Do companies like Oracle and SAP run their own business on their ERP platforms? That might be the biggest one.

How is facebook dogfooded? Do they buy facebook ads?

And wasn’t windows originally created as a product for sale? I don’t remember microsoft starting as hardware company. Maybe that is why windows sucks :)

None.