Would you consider Anakin to be the main character of TPM? I wouldn't.
Who would you then?
I don't know. I've always thought of TPM as the movie without a main character. In Episodes 1 and 2, Anakin is the main character. In the whole OT, Luke and Vader are the main characters. But as I said, TPM has seemed to be filled with "main" characters (such as Obi-Wan, Qui-Gon, Maul, Anakin, etc.).