This is how I see the ranking from strongest to weakest. Its not based on 21 levels or something. Just the first in the line is stronger than the second and goes on like that...

Female:

Lanfear/Alivia/Cyndane/Graendal/Sharina/Someryn(?)/Nynaeve/Semirhage/Talaan/Mesaana/Moghedien/Tamela/Viendre/Egweene/Elayne/

Aviendha/Bode/Cadsuan/Nikola/Kerene/Meylin/

Male:

Rand(LTT)/Ishamael(Moridin)/Aginor/Demandred/Sammael/Rahvin/Logain/Taim/Balthamel/Be'laal/Asmodean

 

 

There's actually an official ranking, maybe a teaser from the Encyclopedia and I believe Lanfear is 1 and a ? is number. I think Fortuona is number 2 but we'll never know now.

to the best of my knowledge,aviendha is on a par with elayne and egwene.

I believe Talaan is stronger than Nyneave

Some of those are on the same level like Egweene and Aviendha, but i believe Egweene is just a hair stronger. I think that Nynaeve is stronger than Talaan even by a hair too. We don't have a single quote that Talaan is stronger. It is always "come closer" or " there is almost no difference" or 'almost as strong as Nynaeve"...

the Thirteenth Depository site has a page that lists the saidar ranks.  directly below that list, they tell their reasons.

 

the upcoming Companion might tell Robert Jordan's lists.