I would recommend Ryze, Sion or Gragas. All 3 play very different so you can pick depending on your play style.
Ryze is nice because alot of his kit scales off of max mana. You can build tanky items with alot of mana on them and get AP. So, later in the game you are built like a mage-tank with Frozen Heart and Banshees Veil.
Gragas is good because he is really versatile. Some great poke/safe farming with his barrel. His body slam/charge ability allows you to dash through terrain. The bosy slam is really good for chasing and escaping as well. His ult is devestatiting in team fights, helps in dragon and baron fights, and can knock away enemy champs to help escape. Gragas can also clear waves incredibly quickly allowing you to leave your mid lane to assist in ganks.
Sion is good for this same reason. He can pop his bubble/shield on a creep wave and then just roam with +5 boots. The more AP you have the bigger your bubble gets which makes you pretty durable as well.
Out of those 3, Ryze is probably best late game. Sion is probably best early game and then will fall off later in the game. Gragas is pretty well rounded early late.
i recommend all 3 if youre trying to raise elo as ap mid.