What keystone should i use for jhin?

Use aery. Lethal Tempo doesnt help much i think and fleet footwork is just a weaker warlords,for those that think otherwise there's a huge buff coming for it next patch,so its kinda obvious it was falling behind with the other masteries

Comet is what most pros are building.

Most people use Comet. I've seen Electrocute work pretty well too.