If you're going to choose between 7 and 10 I'd say 10 because it's
- A completely redesigned kernel and core features
- Has much better built in security and health checking
- Longer support life
My real answer would technically be some flavor of Linux server edition since it's not as susceptible to malware, free, and it uses much less resources to manage the OS itself than any edition of Windows currently; however Windows 2016 Micro servers may be able to compete on the small footprint. You can run .Net applications on Linux provided you have Mono setup.