The whole question fits in the title. And to add some context: I'm not asking what is the best according to what the specs are saying, but rather what works the best given the mix of browsers deployed nowadays.
Some data points:
text/javascript
for the JS used on their home page.text/javascript
on Google Docs.application/x-javascript
to serve JavaScript files with their Ajax libraries service.application/x-javascript
to serve their JS.application/x-javascript
for the JavaScript served on their home page.According to the IETF's ECMAScript Media Types Updates as of 22 February 2021, the RFC-4329 is obsolete.
Therefore:
text/javascript
is a recommended standard (both by IETF and by MDN)application/x-javascript
was experimental while deciding to move to…application/javascript
is obsolete