I love Windows 7 (hate Windows 8, but that's a different story), properly configured it's what I install on all our business PC's. That being said, for older systems that can't run 7 I install Ubuntu. It's the easiest to setup, most user-friendly (in my experience), and has the best online support forums for when you need help. A basic Ubuntu install will give you everything you need (latest versions of LibreOffice, FireFox, music & media player software, the only thing I install on top of that is a screensaver package since the default install includes no screensavers. Throw GIMP and VLC on there and most users won't need anything else).
Your mileage may vary.