Just a thought that I've been considering, but I think it might be Twitch in my opinion, I mean he's just an overgrown rat with a crude crossbow that's going to backfire and kill him one of these days.
The weakest champion would be one that relies on machinery/a firearm that does not use personal agility/strength to fight. Among those considered would be Rumble, Graves, Ashe, ect, however some of these champions use personal agility alongside the use of a simple weapon (graves dash, vayne tumble). Rumble relies on his machinery but he shows he has personal agility with his taunt. What AD carry does not have an escape or magical powers or use any mobility? Miss Fortune and Ashe. Between those two, Ashe has more presence in the world so she is not "weak" as she is a Queen with many supporting her. Miss Fortune has no back up, uses a gun, and does not use her personal strength to fight at all.