Rinnegan is the most powerful eye technique because, the clan that bore the Rinnegan was all murdered just like the Uchiha clan has been. The Rinnegan is so legendary because it can raise someone from the dead (impossible!), sense someone's chakra and absorb it all which kills them, read minds, see from any angle, see through other people's Rinnegans, kill someone if given eye contact, learn ONE of the Pain's unique jutsus (if strong enough), can control the minds, bodies, and souls of other people, can release someone's soul, haunt someone's body forever, take away all the opponent's chakra natures and keep them as your own... There are more to be found!