Dear (friend),
It is such a pleasure for me to be writing you this email regarding my favorite subject in my school, which is (subject). This subject is outstanding. It not only helps me understand (the purpose of the subject), but it also helps me in (extra activities the subject has brought to you). In my honest opinion, it is one of the best subjects because (3 reasons). All in all, you've got to tell me if we match!
Yours sincerely,
(name)