Sharina is said to be stronger than Nynaeve, but Alivia is considerably stronger. I've always believed that Alivia is two levels higher than Nynaeve, but I don't agree with Fionwe that Alivia is as strong as Lanfear.

For me, it probably looks like this...

Lanfear
Cyndane, Alivia
Graendal, Sharina
Semirhage, Mesaana, Nynaeve
Moghedien

Most of what we know sort of confirms this, and Graendal's statement on strength is a vague reference to Lanfear being the only one of the Chosen who exceeds her, sort of like an irritating fact she tries to deny. I don't know how strong Mesaana is, but Sheriam's reaction did surprise me. On the other hand, I also wondered if it was Cyndane who appeared to her - because she has been running around doing Moridin's work in most places, and her orders do countermand Mesaana's.