"Its power grows with time... As does mine" is not a quote reffering to Kindred. He says that upon buying Rod Of Ages, an item of which power literally grows over time. There is no reason for that to be a quote about Kindred.
Next - where did you get the idea that Karthus is completely immortal? Because he is not.
He is just another Black Mist undead - an undead that can be banished by the likes of Lucian. In other words: he can be completely destroyed with magic or the right weapon.
Mordekaiser's undeath is unique in that he can be raised no matter how many times his spirit is banished - That's why he needs the liches and the bones. It's a higher level and a different type of undeath. His lore also specifies he is far more powerful than any spirit from the isles - that includes Karthus.
"in the end only dead shall remain" refers to the fact that one day the world will end. It may take milions of years, but the fact is: it will. And when it does, everyone will die.
It isn't a power declaration, it's not him saying "you cannot stop me, one day I will turn everyone to my side". No. It's just him stating a fact.
Karthus is probably one of the most powerful Black Mist undeads in one aspect - he retained his own will and mind, unlike most, for example Hecarim, who became more twisted.
But in terms of raw magical talent... Well, I don't think he is all that amazing.
He didn't show any magical prowess before being turned, so his abilities come only from the black mist - as such the reach of his powers is limited and akin to others like him: mainly Thresh, who also didn't possess magical abilities before being turned.
His determination may make him a better mage - him trying to further his powers is a possibility... but I still think he is of a similar power level to all of the more important shadow isles denizen.
Comparing him to Kindred isn't valid, I'm afraid. As some others have said, Kindred is the literal personification of Death in runeterra. We don't actually know their extent of power, but it's far up there, in the godlike realm.
Let's say it that way:
Every strong undead got a power-up from the mist. Those that had magical or physical power beforehand are now the most powerful.
This can be seen with Mordekaiser: he had both vast physical and magical abilities, so for him the power he gains from the mist is "considerable" (as stated in his lore). Considerable as in: power lesser than what he already had, but still worth taking note of.
Karthus, who had no magical or physical prowess gets just that power up, and that's the extent his powers reach.
The same with Thresh.
Hecarim was already a knight and was at least somewhat strong, so he probably ranks higher than Karthus in terms of pure power, but his mind was weaker, as he became more twisted and cruel - mist could influence his mind.
Hope that explains the power-ranking somehow.
We don't know how powerful are any of the Mist undead, so it's problematic to compare them to anyone from the outside.
But just based on the current knowledge we have, the power ranking of Shadow Isles champions would present as such:
(there is quite a bit of a power gap between him and the rest.)
(if he continued to extend his power after turning - if he didn't he is where Thresh, etc. are)
/ 
Kalista, Yorick and Maokai are the hardest to place.
Kalista is probably at the same level as Hecarim, maybe a bit higher.
Maokai is a spirit of the forest - we cannot measure his strength in any way.
Yorick is a human, but he has some power over souls there, so yeah. He is probably a bit lower than Karthus.
EDIT: Some points that have been brought up by Eternal791:
Thresh and Kalista consume souls to gain more power, as such they probably rank higher than I thought.
Karthus probably doesn't do that, since he wants souls free, as such he probably falls under Thresh and Kalista in the ranking.
Hecarim is confirmed not to care about any of the souls, so he is weaker than Thresh and Kalista lore-wise.
