Best: Gaga (If you can call her from the new generation), Little Mix
Worst: Selena, Demi, Ariana (She has good vocals but she has a potato's stage presence)
Most of them lack stage presence which is really important. It is something that Selena, Ariana and Demi all lack regardless of vocal performance, dancing skills, etc. I think the girl groups are better in terms of presence onstage.