Hermione's favorite class was Arithmancy. Since Hermione was a prodigy and had a great interest in logical disciplines that could be mastered through focus and interest, she really liked how interesting Arithmancy was. Arithmancy dealt with calculating and plotting possibilities and predicting the future based on events, numerical data, and star charts gathered from the past. As opposed to Divination as taught by the seer Sybill Trelawney, which Hermione loathed for being an imprecise field of study, she loved Arithmancy despite how difficult it could be to study.