Leblanc. Her scaling into late game is just really weak, so if you hold out and can grab her bounties you can make a strong comeback. Same goes for many AP assassins. Building to withstand the burst and then into offense counters fed AP assassins pretty well. Not so much AD assassins though.
I'd say scariest fed on the flipside are infinite scales. Nasus, Veigar, even Cho'Gath. Once a Cho is too far ahead it's game over.
As mentioned above, also Teemo, he relies on you to attack and chase unprepared.
I'm terrified of a fed Hercaim though...