Who was the first Pope?

The Pope is the head of the Roman Catholic  church.

Doctrinally the pope is regarded as the spiritual successor of the apostle St.Peter the first pope: In Matthew 16: 18-19 Jesus says "and I say to you that you are Peter and on this rock I will build my church." However it took  more than a thousand years for papacy as we know it today to be formally institutionalised. 

Historically speaking pope Pius I (142-155) was the first single bishop presiding over the diocese of Rome.

Gradually the power and the influence of the  of the pope increased as the later popes  involved themselves in temporal and political matters: In 452 AD it was pope Leo I who went outside Rome and met Atilla the Hun and convinced him to withdraw his forces. In 800 AD pope Leo III crowned Charlemagne as the first Holy Roman Emperor.

But it was only under pope Gregory VII who was elected in 1073 that the pope virtually became a monarch. Historians treat the period of pope Gregory's rule as the apex of the papacy.

Needless to say, the instituion of papacy and the role of the pope is constantly changing and evolving, for in history only change is constant.