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.
Fed Yi right clicks you, Qs when you CC, W to tank big damage he can’t stop. Nothing out DPS a 6 item yi, and with his true damage armor helps much less. Pretty sure he beats jax if he Qs the counter strike stun.