I really like malz mid. Unless there is a kass/morgana/galio/akali. But pretty much you Hit e on target, then w under them. then R and they die.. Easy lanes and ganks are really strong when ult is up.

Easiest champions to learn mid are Annie and Morg

Annie has AOE skills stuns, great at farming
Morgana is farm with pool, in teamfights press R and Zhonyas

Like everyone else here, I would recommend picking up Annie. I always recommend to someone who is interested in picking up a AP carry. Simple mechanics and heavy burst early game makes her feel rewarding while you are picking up. It will also help you understand the presence of a strong early game AP and their impact on other lanes and jungle.

Annie, Morg, Sion, Ryze, Vlad, Morde

Ahri is a great choice to learn, even with the upcoming nerfs. Very safe pick, never really a counterpick imo but can has potential to outplay almost any matchup assuming superior player skill.

malz is fairy easy as well.

Try AP soraka.

twisted fate.

AP Yi is the easiest by far. Sustain, easy farm, easy nuke.