I think it's important to consider not only what powers each character is capable of, but also what weaknesses they may have and could be exploited. In particular, I'm talking about Terra and Sora, the two characters who have failed the MoM for their susceptibility to darkness. Let's say that Terra and Aqua were equal in strength; I'd still personally rank Aqua ahead because she has one weakness less.
Now, if I had to choose between Aqua and Riku, whose survivals have become testaments to their mastery of their weaknesses, I'd be more inclined to choose Riku over Aqua. Not really a choice I'm even remotely sure of, though; it's between Aqua's feat of holding up to Terra-Xehanort and Riku's dual-wielding and harboring of XH's power (as Rikunort, anyways).
Now, if I had to choose between Aqua and Riku, whose survivals have become testaments to their mastery of their weaknesses, I'd be more inclined to choose Riku over Aqua. Not really a choice I'm even remotely sure of, though; it's between Aqua's feat of holding up to Terra-Xehanort and Riku's dual-wielding and harboring of XH's power (as Rikunort, anyways).