I remember somewhere that Athel used to be to overpowered of a starter champion so they nerfed her but buffed the rest. I don't think that you can openly rank all of the overall too well, because they have different strengths in different areas. If I were to try to rank them overall, I would say Kael, Athel, Elhain, then Galek. They are designed to be even so someone who picks one won't be considerably behind someone who picks the other. If you were to rank them in specific areas; however, the rankings would vary. For example:
Campaign: Elhain, Athel, Kael, Galek
(Elhain has the most AOEs, and Athel has a reduced cooldown with Extra Turn)
Dragon: Kael, Athel, Galek, Elhain
(Ordered by their helpful Debuffs)
Fire Knight: Athel, Elhain, Galek, Kael
(Multi-hitters are best plus some bring important Debuffs)
Spider: Athel, Kael, Elhain, Galek
(This one is tough to rank due to each of them not really having the key components to take the Spider down. It comes down to AOE damage and important Debuffs)
Ice Golem: Kael, Athel, Elhain, Galek
(Poison is great for Ice Golem, and I like Athel because of her Weaken and buffs)
Arena: Elhain, Kael, Athel, Galek
(It's close here. Kael is nice because of turn meter off kills, but so is Elhain because of double AOEs)
Clan Boss: Kael, Athel, Galek, Elhain
(Most to least important Debuffs)
Potion Keeps: Athel, Kael, Elhain, Galek
(Most to least utility and damage)
Taht is my overall insight. Feel free to comment if you think I am mistaken!