Itachi is a prodigy. More naturally skilled than both obito & sasgay combined from the start. Being trained by madara & taught more uchiha techniques &/or even the little things would do just that much more to show his superiority over them both.
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Itachi would show the most potential out of the three, especially if he wasn't scarred by the events of the 3rd great ninja war. Sasuke on the other has greater chakra reserves compared to Itachi and may utilize Madara's training better. We already seen what Madara''s influence has done for Obito.