As Shannow said, we've discussed this quite a bit, so have a look at all that.

Some specific points:

1) Cadsuane, in some entries, is level 9, equalling Metarra, Bode and Nicola (Bode is a guess, but nothing else can possibly fit). In others, she is level 5. RJ changed his mind on her, but nothing in the books shows this. It appears Brandon was working under the assumption she was level 9, too.

2) Aviendha: Terez has confirmed that level 11 was only her current strength, not her potential. This is also Elayne's strength as of WH (she says Tamela was stronger than her, and the Companion says Tamela was level 10). Aviendha will be fully as strong as Egwene was, eventually.

3) Rahvin being strong as Ishamael is a surprise, but remember... this only emphasizes that actual OP strength is only one part of the story. Rahvin certainly wasn't as skilled as Demandred, Lanfear, Graendal, Semirhage and Moridin. Sammael may be in that list too, but those five, regardless of OP strength level, seem to be the best with the OP among the Forsaken.