From what I hear, Vayne is the best choice. True damage, mobility, and a brief stealth. Seems legit to me.
Personally though, I really like Miss Fortune. I don't really use her Q, but I really like the rest of her kit. Her W stacks up Magic damage on each hit, so it works VERY well with a Wit's End, to make even those with Armor take a lot of damage from your auto attacks. Her E cuts off pursuers and helps you quickly gank people with the AOE slow. Her ultimate can be a major game changer. If the other team forces a team fight that your team doesn't want to fight yet, your ult can force them out of the lane, or very far backwards. It also MELTS minion waves. So, god forbid, a very large minion wave stacks up, one ult can clear it near instantly. Plus, the passive movement speed boost is always nice and gets you help in last hitting, as you can back off quickly before the other carry can harass you. But she is just my preference.