The short answer is, almost anything that is currently supported and that has a legitimate license key. We’ve made mention of it before but Microsoft is offering free upgrades to Windows 10 this summer to any user of Windows 7 or Windows 8. The exact version you get depends on what you are upgrading from and Ars Technica has a good read on that below:
If for some reason you are running Windows XP or Vista, you can’t do the upgrade. Really, though Vista is still in extended support, you should not be running these systems anymore, just do the age of the hardware they would be running on.
Once we have a chance to do the upgrade, we’ll be giving out report and recommendations on the process. There is no rush to do this, as windows 7 is supported until 2020 and Windows 8 far beyond that. It’s is a good perk to consider though if you have the confidence to try it out.