Depending on which group it is, this "favorite voices" thing is going to be hard for me lol.
1.) Morning Musume: Sakura and Miki. Sakura's clearly the best singer in the group so everytime I hear her voice, it's pleasant. Now for Miki, I'm not leapfrogging her from Masaki, Mizuki, Duu, Riho and Zukki when it comes to the "best" voices, but everytime I hear her voice, it is always pleasant for me.
2.) C-ute: Chisato ALL THE WAY.
3.) ANGERME: Meimi, Takechan and Kana. Meimi is one of the best singers in H!P at this point, I have always loved Takechan's voice, and I love how deep and dramatic Kana's voice can get in some songs (Otome no Gyakushuu, Nana Korobi Ya Oki)
4.) Juice=Juice: Sayubee and Kanatomo.
5.) Country Girls: Momoko and Manaka.
6.) Kobushi Factory: Wada Sakurako. I love her voice so much!
7.) Tsubaki Factory: This might sound a bit strange, but I actually really like Yamagishi Riko's voice. Her voice compliments her shy puppy look / personality.