Rydia's tied with Terra as my favorite Final Fantasy character, so I gotta vote for her.
My fav. male in FFIV is definitely Kain.
I can't help but love Rydia. She has a complex personality which can be solemn and caring or tough and self-confident depending on the situation. She is both the cutest and sexiest character in Final Fantasy IV since she appears as a cute 8 year old girl but comes back as a sexy woman later on. Not to mention her awesome skills with black magic and summoning. That and she totally put Edward in his place after he was being, for lack of better word, emo, after losing his fiance when she herself had just lost her mother not long ago.