OKAY.. lets leave it at this. Fighting ability does not consist of pure strength alone, but instead of tactics + strength + all that other good stuff. Tactics coming from the intelligent side (aka shika). So, therefore Shika with his intelligence as his aid (instead of power), is a very good fighter. Also, in decision making, the hokages make decisions that would be for battling purposes mainly (whether it be against a village, individual, or monster) and such. And that decision making ability is also an aid in fighting, which highers his fighting ability. So overall, if youre good at fighting, youre a good ninja and the "best ninja in the village" is chosen as hokage based on his/her "fighting ability".