I'm a brony, MLP is my favourite show but Adventure Time is a very close second! I love AT because it's so surreal and dreamlike. much like MLP:FIM It has a lot of subtexts and in-jokes which might go over the heads of younger viewers.
Marceline is my favourite character because I love her personality, If she was real I'd like to hang out with her and form an awesome band (Ice King would be the drummer obviously!)