Xubuntu, for sure. The desktop is easy to use, it's not introducing new concepts like the shell/search environment and everything should be usable via the start menu. It's also reasonable fast on older hardware. I replaced a Windows PC with Xubuntu at my parents in law a few months ago without any problems. If you have technically adept people you could also go for Ubuntu, but I found the gnome shell to be a too new concept to go without problems.
Xubuntu, for sure. The desktop is easy to use, it's not introducing new concepts like the shell/search environment and everything should be usable via the start menu. It's also reasonable fast on older hardware. I replaced a Windows PC with Xubuntu at my parents in law a few months ago without any problems. If you have technically adept people you could also go for Ubuntu, but I found the gnome shell to be a too new concept to go without problems.