Mickey is probably stronger than Sora. Mickey has been wielding the Keyblade even before BBS. He became a master the right way in BBS, and then had another 11 years after that of using the Keyblade. Mickey's power is evident in KH1, since he went into the Realm of Darkness all by himself then he must have been pretty powerful to just dive into the realm of darkness with countless heartless living there.
The question is which is stronger between Sora and Riku? Sora's no weakling not by any stretch of the imagination. Sora's taken down XH, Xemnas, most of the organization, and tons of Disney villains. However we must remember most of the time Sora had help from others.
Still though in KH1 he was able to defeat Riku alone when Riku was under the control of XH. Riku probably got stronger during the year between KH1 and KH2, while Sora slept. So that could have made Riku stronger than Sora by the time of KH2. Sora was obviously stronger in KH2 than he was in KH1, so in my opinion, I think Sora and Riku are about equal.
EDIT:
I'll just give a ranking of where I think each wielder stands in terms of power starting from the strongest and going down to the weakest:
1. Yen Sid
2. Xehanort and Eraqus
3. Terra and Aqua (Aqua being a little stronger now since she has been in the realm of darkness for so long)
4. Mickey
5. Vanitas (Slightly stronger than the others on number 6 but Ventus is probably on Equal footing with him)
6. Riku, Sora, Roxas, Xion, and Ventus
7. Kairi