logically, it'd have to be sakura. Although sasuke is a fighting genius, and naruto is a master when it comes tactics. Naruto and sasuke wouldn't listen to each other if either one were a leader. You already see how naruto reacts when sasuke commands him. They both have leader personalities. I believe when you have more than one leader in one group, who usually have inflated egos, the one who usually does the following, usually ends up leading..
sakura kind of brings them together...keeps their egos in check, you know...a womans touch, you could say. Not saying all chicks are like sakura, though.