How to diagnose a 500 Internal Server Error on IIS 7.5 when nothing is written to the event log?

Take a look at IIS7's Failed Request Tracing feature:

Troubleshooting Failed Requests Using Tracing in IIS 7
Troubleshoot with Failed Request Tracing

The other thing I would do is tweak your <httpErrors> setting because IIS may be swallowing an error message from further up the pipeline:

<configuration>
  <system.webServer>
    <httpErrors existingResponse="PassThrough" />
  </system.webServer>
</configuration>

If the site is written in Classic ASP then be sure to turn on the Send Errors to Browser setting in the ASP configuration feature:

And finally, if you're using Internet Explorer then make sure you've turned off Show friendly HTTP error messages in the Advanced settings (though I suspect you've done that already or are using a different browser).

In my case:

• The Event Log was empty.
• web.config wasn't corrupt - verified by using same on local machine / using inetmgr

Finally...

• Checking IIS logs showed a request like this

...Chrome/57.0.2987.133+Safari/537.36 500 19 5 312

The key being:

sc-status sc-substatus sc-win32-status 500 19 5

which with some googling pointed me to the IIS_USRS not having read permissions to the www folder