The thing is that power is relative.
Kindred is literally death,from what we know,they can't actually be killed or defeated for good by anyone.
Even Sol or the undeads of the shadow isles will die one day,thus essentially loose to Kindred.
However Kindred is likely bound to lots of rules,they can basically only "attack" you if you have a death experience(or near death),so its not like they could come out of nowhere and kill someone for no reason at all.
Aurelion Sol is pretty likely the most powerful in terms of destructive ability,i mean he can literally destroy the entire planet if he wanted to and its even implied that he is quite powerful compared to his brethern.
And Bard is rather difficult to rank as we don't know that much about his abilities.
He is likely unmatched in terms of traveling power(ok maybe Tahm Kench could beat him to that) and wisdom,but I kinda doubt that he has that much offensive power(as reflected in his gameplay),so I think he might not be as powerful as some believe,instead he might just know really well how to do the right things at the right time to get the results that he wanted.
So in a way each of them have their strong points,Kindred is more or less unbeatable,but has to follow the laws of nature,Bard posesses immense wisdom and Aurelion sol posesses immense power.