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.
It depends, are all champions at full items?
If so, I’d probably say Vayne, Fiora, Camille, Kayle or Gangplank. The amount of mobility damage and self peel they have is just too good. Nothing can beat these 3 late game, except each other.
GP has less mobility in comparison, but his damage is way above the rest and he has access to more range, which lets him join the fun.