I am after recommendations for a framework (or project template) for rapid application development using C# on the back-end. It must support the following:
Would appreciate any/all suggestions/opinions. Thanks Tim
Like anything in software, there are so many ways to do this. You are essentially asking for a complete system architecture. You could try to be more specific, but your question is going gather opinionated answers and this will probably end up getting closed.
But I'm bored and will throwin a couple of pennies before that happens: C# most likely means you'll be going .NET on IIS (though maybe its docker on linux?? refer back to my first line above)
User login/authentication
If its IIS, you could be running Windows Auth, or if in an enterprise, Kerberos or Federation. Or if its going against social sites, maybe OAuth? This one depends on what authentication your users need and is really separate of the other areas below.
SPA / Responsive client / Easy to understand...
You listed some very popular frameworks (Angular/Knockout) used in modern SPAs. You may not like them but these are becoming industry standards. Responsive web is pretty much bootstrap or foundation. Though, angular material seems to be gaining popularity too.
Easy DB access.
Whats your definition of easy? Looking for an ORM? If so, Entity Framework is popular in .net apps. Or if you are more of an SQL person, maybe LINQ is easier. Many options here too.
Flexibility on the client ...
Though you may not like some of them, using popular frameworks means you get a lot of developer support behind it - and a lot more answers here on SO if you have problems. One man's sunset is another man's sunrise. What you consider difficult might be viewed as easy by another. Hard to say whats most flexible but going popular means you got more help.