There are two different versions of this: a Hello! Project-specific version and an all-idols version. I am going to do both when there are two different questions, because I love H!P forever, but other idol groups have been catching my eye for a while, too.
Day 1: My Favorite Idol
Aw crap, we have to start with the hard one. I love lots and lots of idols, but the one that sticks out most in my head right now is:
Sayumi went from being my least favorite to my most favorite fairly quickly. I strongly disliked her variety character, which is why many people dislike her to begin with, but she was the first one in Morning Musume whose name and face stuck with me. And there was a reason.
Sayu isn't the narcissistic character on TV (though she would have every right to be!!). She is so much more complex than that. She is a kind and caring person, a great senpai, and stunningly beautiful. I have almost all of her photobooks, and I hope to get all of her DVDs too.
Runner-up: Tokunaga Chinami