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.
Depends on stacks
The more stacks Bel’veth has, the more champions she can fight 1 vs 1
For example, Yi starts getting out-stat checked when she hits around the 150~200 stacks
But then again, that’s every infinitely scaling champion