Best scaling AP mids with new itemization

cass does mediocre dmg even with 900 ap.

new itemization means zilch for late game AP carries

If you want the best ratios, that would be Fizz with a Lich Bane.

Build full AP on him with Lich Bane, activate W, and Q. Urchin strike(Q) procs and applies Sea Stone Trident (W) and Lich Bane instantly, with an additional 60% AP scaling and 100% AD scaling. Sea Stone Trident has around 40% AP damage when activated plus % target missing health.

Fizz is commonly considered by the player base to be the best assassin that can function without an ult, because in one strike, he can deal more damage than other champions in their entire rotation. Plus, he has his playful/trickster with a 5 second CD at max cdr for easy engage and escapes.