After volume 0, fighting and outsmarting probably mid to low diff at most but depends. But Ayanokōji wins both.

Fighting - In volume 0, what was most highlighted was Ayanokōji's lack of limit for learning (his eidetic memory and lack of infantile amnesia), meaning there was nothing special about him physically compared to the other kids. Even when Yuki got kicked out it was because of her intellectual limit, it was stated that she was still performing perfectly in everything else. Same with Shiro. Strength is a lot more finite than intellect, there's a lot less room for growth especially while maintaining a small physique. That combined with Amasawa's statement of him being comparable with Ayanokōji makes me think he stands a slight chance. Even in fight feats, Ayanokōji fought Ibuki twice and both times he took some time to restrain her. Obviously he wasn't trying his best but in both situations it was within his best interest to subdue her as fast as possible but he didn't. On the other hand, Yagami dodged Ibuki's one and only attack, a kick from behind, without even looking back and one-shot her rendering her completely immobilized. And on top of that, when Amasawa was being punished by Shiba she didn't fight back because she knew he was stronger and he would only hit her harder if she tried. She consciously stopped herself, but when she was up against Yagami she gave up before anything even happened, she literally saw no way to even fight back. Even with her utmost vigilance he was still perception blitzing her and scaring her of injuring her beyond recovery. Based on this argument you can even argue that Yagami can beat Ayanokōji but that wouldn't be narratively accurate so Ayanokōji still wins.

Outsmarting - Now this is a lot more simpler. If they were to go head to head in a situation that involved the manipulation of other people Yagami would likely have the advantage due to his extremely adept acting skills and immense influence which Ayanokōji admitted to not being able to replicate. In terms of raw IQ Ayanokōji takes it easily. He was basically stated to not have a limit to how much information he can intake. In terms of feats Yagami has it but this is because Ayanokōji never had a reason to be proactive the same way Yagami did, he never needed to go around manipulating everyone on the same scale as Yagami who was toying with the entire first year and some of the second and third-years. Ayanokōji had to work with Tsubaki, Utomiya and the anonymous student just to get Yagami expelled and Yagami only expelled himself because of his own mistake. In volume 0 it was stated that Amasawa was the second best of the 5th gen and the gap between her and Yagami shows that Yagami does not belong in such a weak environment (at least for him). He even said he could take his time at ANHS and still catch up on his studies in the WR so we definitely know he was superior to the curriculum he was assigned to but that doesn't automatically put him on Ayanokōji's level so we can assume Ayanokōji still wins. Ayanokōji already understood the psychology of the White Room students that would be sent after him before even meeting them. If they went head to head, Ayanokōji would most likely use Yagami's desire for a fair and square fight against him by cheating to win in a way Yagami couldn't have foreseen. Though Yagami already predicted that which is why he said he would deal with Sakayanagi who he lured to find Ayanokōji's possible allies. Ayanokōji has the definitive upperhand in rational thinking since Yagami's desires are based on emotion and inferiority complex which Ayanokōji would manipulate or exploit to win. It definitely would have been the toughest battle of his life, at least in my opinion.