Rabbeinu Gershom, whose takkanot are considered binding by many Ashkenazim, is often the earliest name which I hear being cited in relation to the Ashkenazi mesorah. From whom was his mesorah, if known? Can it be traced back to Chazal?
From this article on early roots of Jewish life in Germany, it sounds like Rabeinu Gershom himself wrote about Rabbi Yehudah ben Rabbi Meir HaKohein (aka Rabbi Leontin) in one of his teshuvos, that it was "...Rabbi Leon who taught me the majority of his learning."
The Shaim HaGedolim from the Chida brings three opinions regarding who his teacher was. One is Rav Hai Gaon, another is Rabbeinu Chananal, and the third is Rabbi Yehuda Ben Meir HaKohen HaZakein who is called Rabbeinu Leontin.
The entry for the latter has no biographical information, but under Rabbeinu Leontai it says in the notes that this is an Italian version of the name Yehuda, and he is referenced that way in Hagaos Maymonis.