I purchased the Companion, and it has a the majority of channeler's strength in the Power levels.

I guess some of us have been debating this for at least a decade. Many of the levels could not actually be worked out, if you look at them, but it's nice to know how it all fits in.

Levels

Before the wonder girls arrived there were levels 1-60.
After they arrived another 12 levels formed on top.
List is therefore 1(strongest woman possible) to 72 (weakest woman possible).
On top of this are 6 male levels designated ++6 (weakest) to ++1 (strongest man/channeler possible).

The Forsaken

Ishamael - not officially mentioned. Companion says he's the strongest of all the Forsaken. Moridin is ++1.

Rahvin - ++1
Demandred - ++2
Aginor - ++2
Sammael - ++2
Asmodean - ++3
Balthamel - ++3
Be'lal - ++4

Lanfear - 1
Semirhage - 1
Mesaana - 2
Cyndane - 2
Graendal - 3
Moghedien 4

notes - I think my biggest surprise here is that Rahvin is the strongest male Forsaken alongside Ishamael. The others are not really surprising, except that Be'lal was even weaker than Asmodean. The females are also not too incredible. Most of us thought Graendal was higher up, but the list isn't shocking.

Other characters

Taim - not mentioned, nor M'Hael.
Rand - no mention of his strength.

Logain - ++2

Alivia - 1
Sharina - 2
Talaan - 2 (in the books she doesn't surpass 4)
Nynaeve - 3 (in the books she doesn't surpass 4)
Caraighan - 3
Someryn - 4
Cadsuane - 5
Tamela - 5

Egwene - 8
Elayne - 8
Nicola - 9
Metarra - 9
Viendre - 10
Meilyn - 10
Aviendha - 11
Therava - 12

Moiraine - 13 (66 after her release)
Siuan - 13 (35 after her Healing)
Elaida - 13

Soriliea - 57

notes... Many of these are surprising, especially Cadsuane, who is only a small step below Moghedien, and apparently stronger than any of the girls. Aviendha is also weaker than both Egwene and Elayne in potential. Logain is high male Forsaken strength.

In terms of the Forsaken we can see that Nynaeve and Graendal have equal strength. Cyndane, Mesaana, Talaan and Sharina have equal strength. Alivia, Semirhage and Lanfear have equal strength.

None of the Ash'aman are mentioned in terms of strength, and basically all of the female channelers are. The introduction of the character usually starts with "Alviarin, strength 17..."

I guess my biggest surprise in the strength list is that Rahvin and Cadsuane are so strong, but there you have it!