The topici s who had the most potential, not who has grown the most throughout the series. I think the strength/potential of an Uchiha is connected to the pain the had to endure in their respective lives. W/o their pain they would have been significantly less strong. Therefore we should look about their talent before the pain entered their lives. if we look at it that way we will see that obito was completely fodder. i dont want to flame obito fans but if u look at him in the kakashi gaiden arc u will notice he was pure hearted but completely useless. Sasuke was strong indeed. He learnt the fireball technique very quick and seemed very skilled for such a young age. we cant say a lot about shisui tho. on the other hand we've seen a lot from itachi. and that is by far the greatest feat. his shuriken technique is till today uncompared, he had great intel and was in anbu with extreme young age. "pain-wise" i would say obito had grown the strongest therefore should have the most potential, but potential just from talent i would say itachi