For me, if we talk about the safest way to store your Bitcoin stash is in an air gapped computer. And I will prefer to used freshly installed Linux over Windows for that particular device. Why Unix? First, it's over source and then not bloated with apps that you don't need since you will be just using that device for just one purpose only. And it works with old hardware. And why it is safe? It's because air gapped computer should not be connected to the internet, so that attacks are impossible.