Why was Aaron rewarded with the high priesthood when he had made the Golden Calf?
Aaron tried to dissuade the Jewish people from making the Golden Calf; see Ex.32:21-25. The Midrash on Lev.6:2 says that he was rewarded for arguing that the Jews did not deserve punishment. Moses in fact thought that Aaron would not be given the high priesthood because of his involvement with the Golden Calf, until he was told "Take Aaron" (Yalkut Shimoni on Lev.8:2). Incidentally, it has been suggested that the Jews' worship of the Golden Calf was not full idolatry; they worshipped it as an intermediary between them and G-d.
Ask a follow-up question