RJ mentioned Lanfear as the third impressive Forsaken and as the first women among the females:
"Some of those Forsaken the Dark One killed were every bit as high-ranking as the thirteen who were remembered, and who you might say constituted a large part of the Dark One's General Staff at the time of the sealing. With the Forsaken, where treachery and backstabbing were an acceptable way of getting ahead, the turnover in the upper ranks was fairly high, though Ishamael, Demandred, Lanfear, Graendal, Semirhage, and later Sammael, were always at the top end of the pyramid. They were very skilled at personal survival, politically and physically."

This suggests that Lanfear is at least as able as the others among the mentioned Top 6, probably only surpassed by Ishy and Demandred.