While at this moment in time I am slightly leaning toward naming riku the stronger I feel it is necessary to say the Riku himself was close to defeat on several occasions, most notably in CoM, and needed the help of Mickey. Ansem had Riku in his clutches until Mickey helped him in the realm of light and the same situation when Ansem restored him after Sora(Zexion) defeated him. Both have got help from others, its just that Sora has received more as he had more opportunities to do so. For sheer strength I'd say Riku is the stronger, but for perseverance I'd give it to sora. I know both has to persevere hard throughout, I just feel Sora has greater levels of it. I hope they find a way to equal them out in KH3. But I will always feel Donald is the best, so strong, I so never always swapped him from my party due to him having constant death