Vicki Zhao really is the Queen in mainland China
As for Taiwan, Jolin Tsai is doing really well there, her official title's "Princess of Pop". She's also very popular here in Singapore. And she just released a new album with Cantonese tracks on it, so I expect that she's trying to enter the C-pop market as well.
Sammi Cheng is really good, but I haven't seen her around for very long...
I still think S.H.E are the most well-known females in the Chinese market. They're big everywhere
Just that they're a group, so people don't call them "Queens of Pop", like they do to solo artists.