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.
Rn there is not ONE champion who rules them all in 1v1, moreso there are multiples champions that can win 90% of the cast in 1v1 in lategame, and can counter the best 1v1 :
In those I will put (with no order) : Jax, Fiora, Vayne, Kai’sa, Kassadin, Kayle, Master Yi, Nilah, Vladimir, with honorable mentions to Tryndamere, Camille and Yasuo/Yone
In the end, it’s pretty difficult to know who has the best 1v1: for example Yi stomp almost everyone late game, but is very prone to hard CC or AA dodging (Jax, Teemo, Nilah and Shen are famous to allow this).
I would say that if left to late game, Nilah would be the strongest lategame as of now : she pretty much has Jax’s defense with the ability to almost oneshot you with a Q/R (unless you have like 5k health and 500 armor, but even then she still removes 70% of your hp). But again, Hard CC shuts her down.
Jax and Fiora are probably the most consistent : as they don’t give up too much strength early game and can afford to build accordingly to match anyone you face, and they are probably the carry that gets countered the least by hard CC. people debate Vayne but it’s the same problem as almost anyone else in my opinion.