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.
If we factor in tactic in these 1 vs 1 then a Full AP Kai’sa can pretty much beat any thing (i mean technically speaking, she never have to commit just launch W from 2 screen away, and I don’t see many champion catching her from 3000 range)