Which AD assassin scales best in late game?

I would say pretty insane resetting execute. Honestly it is actually ridiculous.

Out of all those champions, only Rengar, Kha and Zed really scale. Pyke kinda, talon kinda, def not shaco, and def not nocturne.

Shaco makes an amazing split pusher late game.

Dark Harvest

Kayn and Kha'Zix

You didn't mention or who both scale very well

- Even without Harvest, the kit and numbers alongside itemization have reinvented Shaco to be a good splitpusher, assassin, or teamfighter late game regardless. Requires more understanding of the game state and champion to pull off very well considering his power paradigm is loaded into inventive uses of his kit rather than 1v1, especially as the game goes on unless high DH stacks to remedy a buffer between skill and knowledge. Scales with experience more than anything really.

- Invisibility and Isolation(even just regular almost) damage become nearly unbearable with jump resets and lower cooldowns. Very good objective control/acquisition and very little risk to kill a champion caught out anywhere on the map [especially late game] even if the team is nearby.

- Red Kayn scales just fine of course, no explanation needed. Blue Kayn is variable on team compositions whether he will scale to be a god or useless.

- Oneshot knifecat is an annoyance. Bone Necklace adds to innate burst as game goes on alongside lower cooldowns on abilities with no costs, even allowing to build GA and a tank item on top of burst or full bruiser -- as well as exceptional dueling in jungle or as a backline flank when a teamfight begins to kill the damage. DH also helps of course to do that much better.