Sasuke had the better development of the series. I think his character is the most complex. The turn of events his character went through, the twists and decisions he made throughout the whole manga... The complexity of the relationship he had with Itachi and Naruto. The emotions he felt in ending of the series, accepting Naruto's friendship, accepting to protect Itachi's will, accepting Sakura as a woman, etc.
As for Itachi, i'm waiting for Itachi's own series, hoping to see a little more of his perspective. I want to see what kind of relationship he had with Izumi Uchiha. That sort of details, can't wait for it to start.