Gaara has proved himself to be the strongest of the former rookie group. He has more raw power than Sasuke, and he has more finesse and control than a Kyuubi Naruto. I think the fact that he was able to be a KAGE promotes this thinking. Kishimoto's not gonna spend much time on gaara's development anymore (since he pretty much reached his prime), so he had to make him very strong. While naruto, Sasuke, and Neji have room to grow, Gaara is pretty much at his peak, and he needs to be since he is Kage. This is the reason why Gaara is the strongest of the original genin, including Naruto and Sasuke.