I would say Kennen. He is a good Hybrid, AD, or AP. His ultimate is a great AoE Disruptor, and he stuns like a ninja. No mana, and shurikens.
I wouldn't pick Vayne, because you might feel bad after. Vayne has NO SKILLCAP whatsoever, and I predict a large nerf in the future. The only skill that Vayne requires, is knowing what items to purchase. And even thats easy.
Swain is cool, he's a mage that has a 6 sec cooldown. the only problem is, your either good as him or terrible. I've seen really epic swains, and horrifying terrible swains. Sort of like Vayne.
And Nocturne, ohhh Nocturne. He's good at facerolling, but he's a bit high skillcapped. He was just recently nerfed, making him harder. Its easy to counter him with some champs... (As Kennen, he ulted me and then I Killed HIM.) So, eh.