Windows XP. The amount of things I can still do on XP today is pretty impressive and shows how little innovation actually happened in the meantime or how little I need any modern stuff. I could also mention some Linux distros, but I still do feel that XP is better than them because it has a UI that makes some sense, it has at least some consistency, unlike open source software which I feel is way too fragmented to be coherent for an end user. It's also old enough to not be caught in endless drama.