Another toughie. Before I get to the real answer, I have to say that I appreciate all idols like crazy. Idols have tough jobs. They work very hard each and every day, training, singing, dancing, performing, on variety shows, etc. These girls work full-time, and if they are underage they still go to school (any idol that ends up going to university as well gets my wholehearted appreciation). In addition, these girls aren't paid THAT well (compared to Western celebrities, at least; these girls aren't rich). Also, unless you're someone like Maeda Atsuko or Takahashi Ai, chances are that you'll have to see other people prefer other girls to you, which has to be a tricky thing for self esteem. These girls are put through a lot, and so I think every single girl who decides to be an idol and sticks with it and the hard work deserves a lot of appreciation.
However, one of the many girls I really appreciate is...
AKB48's Hirajima Natsumi!
Most people do forget, I think, that Nacchan was one of the original founding members of both Team A and Team B. She's probably the least appreciated of the original members who haven't graduated yet, and that's a shame because she really has great energy and is really a fun member. She's also a great member in Watarirouka Hashiritai.
Nacchan is a wonderful member, and I know I appreciate her!