Actually, Sora reclaiming the Keyblade doesn't mean he's strongest. It means he was stronger than Riku when he did, and even then - Riku was stronger five minutes earlier to even take the Keyblade from him.
I do think that in the long run Sora has the strongest Heart, mostly because he lets himself open up to people and draw strength from them, so he has practically an endless amount of power as long as he lets himself be supported. Riku on the other hand when he opened his Heart, it was overrun with Darkness.
I do think it'll be debatable in future games though how they'll be affected, seeing how Sora's slowly yet surely moving towards a mental breakdown while Riku already went through his list of issues during the past games and seems more than capable of holding onto his sanity when Sora is involved, so the scale of powers just might turn, but for now Sora. Definitely Sora.
Kairi doesn't even deserve to be up there, I think, especially since her strength of Heart, how much of it she might has, was never truly referred to in the games.
As stated numerous times above me - more so than her even having a pure Heart, she has a Heart which cannot house Darkness. That is the Definition of a PoH given - ones without Darkness in their Hearts due to their connection to the Realm of Light.
From how the plot described it, it's not even because the girls themselves were so pure and rightous and wonderful - the Realm of Light needed seven maimed Hearts which couldn't house Darkness. How those Hearts were handed out no one really knows, but seeing how it's something set at birth even, I doubt it shows anything about a PoH's strength of Heart.
We'll throw in the Fun Fact then that Kairi was soooo stroooong she kept getting herself into trouble and kidnapped and needed rescuing and you'll see why I don't really care for her when thinking about Strong Hearted people.
If anything, she comes out more stubburn than strong and detached from the reality of her being able to do effectively nothing to help - something she finally came to realize only at the end of KH2, and even then - she was forced to finally see Sora and Riku fight because they couldn't just drop her off somewhere like they did in the past.