Who wrote our haggada? Was is a tanna, amora or reishon? I've heard different rumors from different people. Some say Rashi compiled it. Some say it was a tanna while others say it was an amora.
In the Sefer Hatoda'ah by Rabbi Eliyahu Kitov he writes here and here that from Yetzias Mitzrayim each person told the story to his family to the best of his ablity, except for the hallel that was said in mitzrayim was said and as time went on they added from the neviim and Dovid HaMelech until the Anshei Knesses HaGedolah came and wrote the main body of the Haggada and the brochos as we know it. As time went on the chachomim added and changed certain things in the Haggada such as they took out in the ma nishtana the question of eating the korban Pesach and added the question of haseibah and added the "ha lachma anya".