The title has been changed to accurately reflect what we're really talking about here.
Like it or not (I certainly don't in reference to one group), this are the five biggest - and most impactful - girl groups that have come from America since the 60s. That is why they're being rated.
ftlom - if you wanted to do a true R&B girl group rate - SWV, Xscape, 702, Brownstone, Changing Faces, Zhané, Blague, etc and so forth - you're more than welcome to it.