Which champion is the best duelist?

wheres master yi? a yi thats 0.0.0 with and could outduel all these with 2 items as well.

This is no simple question. Do you mean at full build? Early game? No items? All of those factors change when a duelists power will be. Some duelists are more early game. Some are later game.

I believe Fiora would be the best duelist most of the time, she's designed to be superior 1v1.

just saying that triforce tahm kench could be up there, i know he beats yasuo and irelia

Definitely Fiora. She can 1v1 literally anyone in the late game, if played correctly. Her whole kit was made for 1v1's. I can also tell specifically why: vs. Xin Zhao - late game, your true dmg melts him and you can easily stun him by blocking his third Q. vs. Jax - it's a tricky one. Noone really has the advantage here, whoever stuns the opponent wins, unless Fiora can proc all 4 vitals on her ult really fast. vs. Darius - this is a joke, you can EASILY kill him before he even stacks his passive. vs. Tryndamere - you need to proc 4 vitals on ult fast and kite him with your insane mobility until his ult wears off. If you manage to do that, it's easy. vs. Irelia - Irelia scales really bad into the late game and can't 1v1 any champion on this list. vs. Riven - she is too squishy and you are too tanky for that to even be considered a skill matchup late game. You just deal so much more dmg to her than she does to you. vs. Yasuo - this may be hard if you fail to block his third Q, but its really easy if there are no minons nearby for him to dash on. vs. Warwick - you just simply outdamage him insanely hard. vs. Trundle - your true dmg makes half his ult useless and he simply cannot deal enough dmg to be on par with you.

To sum up - Fiora's kit just counters anyone trying to 1v1 her. You deal 60-80% of your enemies' hp in true dmg with ult alone and you have 45% life steal with 2k ult heal.