Give some stream of consciousness and then the answer. In public, Joffrey is allegedly the oldest son and heir of King Robert Baratheon and Queen Cersei Lannister, both of whom entered into a political marriage alliance after Robert took the throne by force from the' Mad King' Aerys II Targaryen. In reality, his biological father is his mother's twin brother, Jaime Lannister. He has a younger sister, Myrcella, and a younger brother, Tommen, both of whom are also products of Jaime and Cersei's incestuous relationship. Their sole biological grandparents, Tywin and Joanna Lannister, were also first cousins.

Based on this passage, game of thrones who is joffrey's father?
The relevant information to answer the above question is: In reality, his biological father is his mother's twin brother, Jaime Lannister.
Therefore, the answer is his mother 's twin brother , Jaime Lannister.