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.
Fiora is literaly the grand duelist a title well deserved.