Who is a good choice vs AP cho'gath mid?

An AP Kog against him does wonders (I'm by no stretch a kog main, but in the match up's I've seen Kog was most effective). With the % HP Dmg his stacks don't matter - Vayne could do the same, but Kog has the better poke advantage.

Fizz

The key to dealing with cho'gath from my experience if being able to keep him far enough away he can't silence you, while still being able to farm. As such I suggest Xerath or Ziggs.

I also feel like Brand should work well due to his passive, but the silence range still concerns me. I also think that Urgot works fairly well.

I see Ekko doing well because of his %hp damage as well.

Nid, Yas, and Zed seem to work fine against a cho'gath. I'd suggest anyone with a small dash to keep yourself from getting knockedup/slowed.

Fizz, Ahri, and Azir are all excellent picks against mid Cho'Gath.

: can use (E) Playful/Trickster to avoid 's (W) Feral Scream and then (Q) Urchin Strike to completely avoid his Rupture as well. Cho'Gath's immobility means landing (R) Chum the Waters is really easy. This matchup all comes down to proper timing.

: can use (Q) Orb of Deception's speed boost to avoid his Rupture while also being at a save distance to avoid Feral Scream. Ahri is both simultaneously long-ranged and short-ranged, but can close the gap without much fear due to her initial mobility from her Q and R, and circumvent Cho's predictable spells (an AoE skillshot and a cone skillshot).

: can abuse Cho'Gath being a melee by constantly harassing with his (W) Arise's sandsoldiers from a safe-distance, whilst also avoiding Rupture by using (E) Shifting Sands. Cho has to go into melee range to use his ultimate, and Azir can easily escape with E or use (R) Emperor's Divide to force him away.

Most other matchups are generally skill-matchups, but keep in mind that counterpicking doesn't matter if you can't have an effect on the game outside of laning phase or even during it.