LeBlanc hands down. Compare to Annie:
Annie at 18: 985 damage with a 2.25 AP ratio.
LeBlanc at 18: 1177 with one proc of her e (So her instant burst), 1317 with both (With a slight delay) with a 3.26 ratio with a single proc, 3.76 with both.
Essentially, she has higher flat damage with a MUCH higher ratio. She has far and away the biggest burst in the game without outstanding circumstances like a super-farmed Veigar. Her problem is that she doesn't have a big AoE stun like Veigar or the small AoE stun+two big AoE skills of Annie.