Full Magic Pen Ken

Void Staff/Abyssal/Liandry's/Sorc Shoes/Zhonya's/Rabadons

Makes it so that any magic resist the carries on their team builds is useless, and is quite tanky. This works great especially against tanks because they have to build more than one magic resist item if they want to still be tanky, allowing your adc to deal more damage.

Does a lot of damage in teamfights and items like aegis or banner don't help much

EDIT: I've been using this as my main build for ranked games, and it works great. Even if you lose lane you still do a lot of damage, for example, I was 1/6/1 as Kennen against Irelia, I roam mid and i get a triple