What is Correct format of Content-Type Header for JSON

I have recently started working with JSON and at times I see different Content-Type header being used every time.

Content-Type: application/json
Content-Type: text/x-json
Content-Type: text/javascript
Content-Type: application/x-javascript
Content-Type: text/x-javascript

But which one of them is correct, or best? I have heard that there are security and browser issues varying between them.