Well Rinnegan is, without any doubt, the most powerful one of the three.
But about the Sharingan and the Byakugan I'm not sure. Global seen the Sharingan is more powerful in genjutsu and mid-long range attacks, while the Byakugan is the strongest in close combat (if they would stop the chakra flow, the Sharingan would be useless).
Kakashi said that the Byakugan beats the Sharingan in insight, wich means in analysis, reading your opponent(after that Neji started to analyse Hinata) and not in power. However he said this in the anime, not sure wich words he said in the manga.