Sidious's recent thread has unlocked a direct comparison between the One Power strengths of some of the Forsaken, and once again Nynaeve is the key to a definitive answer in this respect.

Without going into a lengthy post, we can use Nynaeve as a measuring stick to demonstrate how Moghedien, Asmodean and Semirhage compare to one another in EFFECTIVE strength.

Basically, it goes like this:

Nynaeve is still significantly below her full potential in tSR, when she EXACTLY matches Moghedien in strength.

At the end of the same book, Rand EXACTLY matches Asmodean in strength. Neither Rand or Nynaeve are at their full strength yet at this point.

Egwene acknowledges in the SAME book that Nynaeve is weaker than Rand at that point in time - although we didn't need Egwene's statement to grasp that fact, given the status of Rand vs Nynaeve in the channeling pecking order from a thematic perspective.

Therefore, we can definitively state that, as at the time of tSR:

Nynaeve = Moghedien
Rand = Asmodean
Rand > Nynaeve

Therefore Asmodean > Moghedien.

Conclusion: Asmodean is without a doubt stronger in the Power than Moghedien. And we are referring to effective strength here, because when Nynaeve and Rand were compared, Egwene could not sense the QUANTITY of saidin he was channeling, but could merely judge the EFFECT that he was able to achieve (simultaneously shielding both her and Elayne while channeling a dozen flows at the same time).

Thefefore if Rand's EFFECTIVE strength exceeded Nynaeve's at that point, then Asmodean's effective strength exceeds Moghedien's.

Furthermore, since it has been confirmed that Nynaeve and Semirhage are VERY close to one another in strength, we can also state without fear of contradiction that the difference in strength between Modhedien and Semirhage is exactly the same as difference between Nynaeve's strength in Book 4 and Nynaeve's full potential strength.

Since Nynaeve is still growing in strength 3 or 4 books later, it seems pretty clear that the gap between Moghedien and Semirhage is considerable.

In fact, as late as POD, Elayne states that neither she nor Nynaeve had yet reached their full potential strength. That is a long time after Nynaeve matched Moghedien in strength.