Everything we know about the female Forsaken is speculation. In fact, there is only one truly indisputable fact regarding their strength i.e. Cyndane is stronger than Graendal. And from this we can deduce that Lanfear was even stronger than Cyndane. Everything else is hypothetical. We have no idea how the rest of the strength tree looks... It's Lanfear, Cyndane, Graendal... and the position of the other three is unknown.

A lot of the speculation comes from the statement that Alivia is considerably stronger than Nynaeve in WH, implying that there is a large gap between Nynaeve's level and Lanfear's (who is generally considered to be the highest attainable female strength). However, Nynaeve was still not at full strength at this stage. The statement is also made by one of the Kin who has infantile strength compared to both of them. If Alivia has a strength of 90 and Nynaeve 88, and Reanne has a strength of 10, then Nynaeve is almost 20% of her strength weaker than Alivia, thus being 'considerably weaker'. It's also possible Alivia is at 90 and Nynaeve at 75... we just don't know.

We know from RJ that Nynaeve is as strong as most of the female Forsaken, and this is generally taken to mean that she is weaker than Lanfear and Graendal, but as strong as the other three. Moghedien is undoubtedly the weakest, both due to her behaviour, and due to the fact that Nynaeve matched her directly in power early on in the series. Brandon Sanderson also confirmed recently that Mesaana is not a very strong Forsaken in his opinion. Semirhage is most likely to be on Nynaeve's level, or close to it.

The most likely strength tree is Lanfear, Cyndane, Graendal, Semirhage/Nynaeve, Mesaana, Moghedien.

I personally believe that the gaps are small, except perhaps that Moghedien is considerably weaker. I think all of them are 90 to 100% of as strong as possible, maybe even 95 to 100%, which allows them to stand in each other's company without outright murder occuring (which Moghedien does not risk unless commanded to by the Nae'blis). A lot of evidence for the large gaps rests on the fact that high strength males seem to be able to overwhelm higher level female channelers from various quotes, but it has never been demonstrated practically, and even directly contradicted. Most evidence comes from shielding which is very hypothetical and we don't know how the two halves of the Power interact when used on the opposite sex.