Lately I've had a blast playing Twitch in bot lane. Nothing quite like unloading on an enemy champ from the brush, watching them run under their tower for safety, and then pick up a First Blood with Expunge! Thought you were safe, didn't ya?!
Though when it comes to a champion that I can lock into and have a blast no matter what, I'd have to say Evelynn. I've loved her since long before her rework, and I've never had anything less than a great time when I pop out of invisibility and chase down my victims as they run in fear from the deadly spikes tearing through the ground!