Best flash for a Nikon F5?

What is the best external flash to use with a Nikon F5 (film camera) ?

A Nikon Speedlight would be the best bet. There are usually several models that will work with each Nikon camera. They are expensive, but the bells and whistles are definitely worth the cost if you're into professional photography. You'll likely be able to use it on other Nikon camera bodies you have or may own in the future if you go to digital media.