Every time a new book comes out, there are a number of new posts comparing the top channelers based on strength in the power. They generally don't make any changes to the previous rankings, and also usually treat strength and battle ability as if they were the same thing. This is not the case. They are obviously related, but there are a number of other factors that are probably more important than the minor differences in strength between the strongest channelers.

Certain weaves will absolutely make one channeler more dangerous. Reversed weaves make it very difficult to be countered in battle, unless the opponent is using a ter'angreal or weave detection. Unless the opponent is strong enough to completely overwealm the user of reversed weaves, it is likely they will be forced to use most of their strength in defense, similar to what happened between Rand and Lanfear. The ability to detect the opposing gender's weaves has the same effect. Someone like Rahvin without that ability would find himself completely outmatched by someone who does.

Angreal, sa'angreal, weapons and ter'angreal defenses will have another large effect. Debates about strength are less important when Mesaana is sitting on top of the biggest collection of *angreal on the continent (discounting ones that are well protected). Since tGS, almost anyone could steal from there so Mesaana has a window to steal whatever she likes.

Ter'angreal like the foxhead medallion will also completely unbalance the scales, and could become the only reason Nynaeve could survive if she ever battles one of the top Forsaken. Weapons like the balefire rod and shocklance are probably only useful for weaker channelers unable to do that by themselves.

The element of surprise can also play a part, and can potentially make Moghedien the most dangerous Forsaken of all.