Why would you be concerned about the version of SQL Server? That should not be dependent in any way on the version of .NET you are using in the application, since all .NET will be doing is making a connection to a database. The important issue for that is what version of driver you use - and as long as that driver is compatible with that version of the database there won't be any issues.
If you are developing an application - you really should use the latest version of SQL Server and .NET. The only reason to downgrade from that point would be to support customers who are not able to support the latest version on their devices.
Developing for 2012/2014 as the baseline - you should be aware that those version are already 8+ years old and out of general support.