According to Jiminy's journal, Riku was the one who was originally to be selected by the Keyblade, but it chose Sora when Riku turned to darkness. But what does a cricket know about the workings of the Keyblade? Riku was able to call the Keyblade away from Sora, but Sora was able to call it back. Which one is the real one? Who knows. But by the end of KH and KH: CoM, it is clear that Sora is THE Keyblade master, not Riku.