Swain - Pretty easy to use. His passive is amazing for mana, so he rarely goes hungry. His Ultimate, as well as his natural amount of health, makes him tankier than a lot of middle lane champions. Although his combos rely on a good skill-shot initiation. Very fun to use and learn.
Orianna - A bit harder to play. You have to be able to micro-manage with her. You have to be able to grasp the concept that your damage comes from one source, and your health pertains to another. She is amazing for her utility: A shield, a slow, a speed boost and a pull-and-stun. (Also very fun when you realize you can shield an ally, and while the ball is on them, activate skills off of them, even if they are out of your normal skill distance.) Harder to use and master, but helps the team a lot more, I believe.
Hope this helps.