Annie and Vlad are the simplest, Vlad is safer but less likely to get a burst kill early on and he has less CC, in exchange for a massive team utility ulti.
Morgana is the safest by longshot, she is nigh ungankable, and you cant make her leave lane for anything.
If you want to get better at being a mid player, Viktor emphasizes all the key points really well. He has a zone control ability that you should use to protect yourself, ranged poke that is a skillshot, and a trading ability, along with an AoE ulti whose CD should prevent you from using it except on sure kills (i.e. no summoners up, no Shen/Soraka). Seriously, Viktor has to play the lane perfectly to get his absurd mid and late game damage/poke potential, and thus you are forced to become a good mid player by playing him.
Pick up AP Sion if you are trying to learn to be a presence on the map. Because mobility boots are such a good pickup on him, and because he pushes the wave with impunity, he can easily make plays in the jungle and in other lanes. Not to advocate Reginald as a role model, but some of his Sion play is very impressive and will teach you how to control a large portion of the map while still farming your lane.