I think it is Jax. I’ve fought against almost every champion over the years playing Jax. Objectively, I can easily say that Jax is the strongest duelist. Imagine that there is only one person in front of you and it is not a champion who gets unlimited power. Unless he has an infinitely powerful rune or item, Jax wins every fight without any difficulty.
There are many things that can give Kai’sa visions: wards/minions. And if she ever hit even 1 W, she will constantly have vision of target due to her passive. Even if the both have vision of each other at 1300 unit range, Kai’sa can spacing very well due to her E. She never really have to commit and her W will eventualy hit. I played both with and against AP kai’sa before, it really really annoying to deal with before it is pretty much impossible to catch her unless you play something like a Rengar or Master yi. And in case of Yi, you waste much of your ultimate time running up to her from 3000 range away