Had he lived, obviously Cao Ang would have been the best choice for heir, just due to being the eldest so to avoid succession conflicts like what happened with Yuan Shao and his sons. However, if Ang is still killed at Wancheng, I'd say that Chong would be the best alternative. Zhi, while intelligent, was lazy, unmotivated, and irresponsible. Not to mention that he had absolutely no experience in anything that would qualify him to be heir. Zhang was only suited to be a general, not a government official, let alone ruler. Meanwhile, Chong was incredibly intelligent at such a young age and had already been frequently involved in state affairs. Granted, he could have grown up and been corrupted like Zhuge Zhan, or fallen into habits like Cao Zhi, and had no military experience, but I do think he was more capable for the position than any of his brothers, barring Ang and Pi.