Leblanc only truly crushes Veigar, but other champions can do even that better.
She has favorable matchups against:
But again, if you're looking for a counterpick, other people are more suitable.
The odd thing with Leblanc is even though high elo streamers have had great success with her, she runs a weak win-rate of about 40-45% suggesting that the common populace plays her incorrectly: framing her as an extremely weak hero.
Or, and this is just my opinion, she may win the lane against more APs, but she would ultimately cost the team the game due to her weak scaling.
Sad fact, LB often wins her lane against a variety of AP champs, but she is pretty much purely single target and in most hands in most cases is simply a weak pick overall.