Wrong, Orochimaru killed two kages (ho- and kaze-kages), which means at least one Sannin is strong enough to be a kage. After Sarutobi died, Juraiya was asked to be hokage, which means he's strong enough as well. But, as you well know, he said Tsunade would be an even better hokage, so she's strong too. On another note, Sarutobi did not choose the Yondaime over Orochimaru because of strength. He said that he could sense the evil in Orochimaru and that he felt it would be best to not put him in a position of power.