Depends on if you're verified and want to get the code for the CDs, what kind of music you like, etc.
My personal favorite songs are Miria's, Anastasia's, Kaede's, and Mayu's. There really aren't any BAD songs, just ones that are kind of forgettable. The only one that I really dislike at all is Akane's.
If you like more energetic and catchy songs, I'd suggest Rika or Miria.
But if you like songs with more of a vocal element, I'd say go with Kaede. Maaaaaybe Uzuki?
The latest batch (Aiko, Koume, Mayu, Anastasia, Chieri) has some really good songs, though.
Sorry this is long, I get passionate about Cinderella Girls songs.