Depends on what you’re talking about:
In matters pertaining to individual parishioners and parish practices, extending eikonomia, issuing penances, giving spiritual guidance, that would be the parish priest.
In governing and directing a monastery it would be the Abbot or Abbess.
Over both of these would be the bishop, who controls how the Liturgy is performed, whether or not someone is allowed to get re-married, who can be baptized, Chrismated, married, or recieve any of the other Sacraments, who is ordained, who serves in his parishes, what is preached, etc.
If something happens within a local church their bishops would meet together and come to a conclusion. If there’s a tie the patriarch gets tie breaking vote.
If something affecting the entire Church happens then a council would be convened, The bishops of the world would meet together, come to an agreement, take the agreement back to their people, the people would agree, disagree, or depose their bishop. The Ultimate Authority is The Faith, or what has always been believed. What the Fathers have said - we cannot disagree with them. Doing so is what results in bishops being thrown out of their thrones.