Ahri' greatsest counter is...

It's simple really, just take a tank with a big in-game size, and ambush her... from the rear (She's particulary weak to that). Garen should be the most effective one vs her because, as we all know, He IS The Lord of Bushes Taric works too, because his beauty far surpasses hers and we know he can't be charmed.. unless there's an Ezreal in the enemy team.