Cassiopeia has insane damage because her E goes on a .5 second cooldown when she hits with poison. Not only that, but her ult has a huge cone of epic CC somewhat similar to Sona's but wider, and not quite as long.
Brand has some of the most ridiculous burst, plus his ult is great now that they fixed the issue of it bouncing to minions uselessly. He's and can turn ganks around with it, which is always hilarious.
Both of them have solid late games as well.
Also, Kennen is pretty powerful and has awesome escapes with his E.