In medieval European society, was the king of the pope the most powerful figure

Neither the king or the pope because the power dynamic shifted between the church and the kings.

The medieval era in the European region:

Inside the medieval era there were a number of issues among the system and the free folk, along with having some tensions with the Church at that time. Just because the Church wanted a religious form of system and was unable to accept any level of amendments inside the rules and principles offered by the religion.

While there were Kings and other power houses who wanted a more free will to rule these lands. And that's when the conflict begin inside the region, but fortunately we can say that there was some religious Kings who wanted a more religious form of system inside the country and that is when the religion's part came into play. The Pope was then provided by some extra capacity to influence on the system of the States. So, we can just say that there were a number of Kings who were far religious then there counterparts that is how the Pope held more power over the system.