Several ways.
1) The easiest is Command-Tab, that lets you switch from any running application to any other. Of course you may have to use the arrow keys to reach the Finder in the application switcher if it was not the previously active application.
2) If you insist on a *single* keyboard shortcut to make the Finder immediately active, you can create a Service in Automator, and add a shortcut to it:

This service will need only one Action, from the Utilities list (Run AppleScript). Fill it in like the example:

Save it.

Then in System Preferences->Keyboard->Shortcuts, add a shortcut for all applications (make sure you type the service name exactly!)

Now you can at any time and from any application press the designated shortcut (in my example, Command-Control-Shift-Option-F) and the Finder will become the frontmost application.