Peter ( aka Simon/ Cephas)
Beheaded by Nero? No, not really. This legend was dreamed up by the mid-2nd century pope Anicetus (CE 156-166) when he became locked in a conflict with the venerable Polycarp of Smyrna. Polycarp had tried to win the argument (over the dating of Easter) by insisting that he spoke with the authority of the apostle John. In response, Anicetus staked a claim to Peter, and Peter, "Prince of the Apostles", trumps John.
2nd century texts known as the "Clementines" had made Peter the "first Bishop of Rome" and 3rd century invention gave him a 25-year pontificate – which made it a tad tricky for him to have died at the hands of Nero but, hey, this is "tradition."
3rd century Church Father Origen went even farther hatching up a colourful plot: “Peter, feeling himself unworthy to be crucified the same way as his Lord, chose option 'B' – Crucifixion upside down!