It depends what you hope to achieve with your Go Pro. I primarily use it - in a basic way - to re-live the session. Of the easy mounting options, the mouth-cam gives the best view IMO:
www.prostandard.com/Otherwise mount it on the nose of your board looking back. That is the best view to watch your footwork and see the path your board took on the wave (if you are surfing).
You will always be disappointed when you see the footage. The waves never look as big (thanks mostly to the fish eye lens).
A couple of weeks ago I posted my Maldives video - you'll see them both in action if you haven't seen them before. (Some of those waves were 1.5XOH and I did not fly 30' in the air - but Go Pro distorts reality.)