How to counter Ahri
Let us take a look on how to beat Ahri. We will look at champions who are strong against Ahri and who are weak against Ahri. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Ahri. You can already win in champion select with the right counters for Ahri. If you want to know how to counter Ahri then you need to take into account the best picks vs Ahri and the worst picks vs Ahri. This Ahri counter data is based on matches played by Pros on patch 14.23 and previous ones. It is important to see whether Pros counter picked Ahri or whether they played as Ahri.
Champions used by Pros to counter pick Ahri
Looking at the matches from Pros (see table Pros counter picked Ahri) we can see that they picked 
Sylas most often to counter pick Ahri. The matchup Sylas vs Ahri occured 101 times. Ahri has 60 wins and 41 losses in this matchup. Learn how to counter pick Sylas to get another perspective on the data.
Lane outcome for Ahri
When looking at Ahri counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Ahri. The lane outcome of Ahri consists of three outcomes: Win, Loss or Draw. Let's take a look of Ahri lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Ahri.
Conclusion
Analyze the available data and learn how to beat Ahri. To master Ahri take also a look on Ahri builds and runes for Ahri. Good luck, Summoner!