I suggest Ahri. Shes really high skill since most of her attacks are skillshots, but she can effectively assassinate carries when played right.

I have a ton of fun playing Orianna, but she is one of the hardest mids to play. If you like champions with high skillcaps, Orianna is for you, her kit is always useful, her autoattacks can hit very hard with her passive, and she can really mess a team up. The downside of this is that she is somewhat team dependent.

I'm also contractually required to mention Karthus. If you enjoy melting half a team with AOE, then Karthus is the camp for you.

If you like Karthus, I will also recommend Cassiopeia. Her DOT is great, she farms easy, and she is an absolute beast in teamfights. She is also a generally safe pick in my opinion, she zones most champs well, and has decent mobility.

If you want a good champion to play in mid Swain would be a good shot, I have gotten really into him lately as he has great poking and an AOE snare, Late game his ult and save him from death and can tear down an entire team with right techniques.

I will also recomend Ahri. She's a safe pick and doesn't really have any hard counters, she can roam due to her high evade, her ganks are brutal mid game, and she's fun to play.
As for Karthus and Orianna they are very high skill cap champions I will agree. I would also put Anivia in that category. Those are the mids that I play including Morgana. Fun factor is Ahri though :D

I recommend Diana, she can snowball, farm wraiths thanks to her kit, and can gank lanes post-6. She can get tanky depending what items you build. She is pretty easy to play and counters most AP Mids.