How to counter Corki
Let us take a look on how to beat Corki. We will look at champions who are strong against Corki and who are weak against Corki. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Corki. You can already win in champion select with the right counters for Corki. If you want to know how to counter Corki then you need to take into account the best picks vs Corki and the worst picks vs Corki. This Corki counter data is based on matches played by Pros on patch 14.9 and previous ones. It is important to see whether Pros counter picked Corki or whether they played as Corki.
Champions used by Pros to counter pick Corki
Looking at the matches from Pros (see table Pros counter picked Corki) we can see that they picked 
Ahri most often to counter pick Corki. The matchup Ahri vs Corki occured 18 times. Corki has 12 wins and 6 losses in this matchup. Learn how to counter pick Ahri to get another perspective on the data.
Lane outcome for Corki
When looking at Corki 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 Corki. The lane outcome of Corki consists of three outcomes: Win, Loss or Draw. Let's take a look of Corki 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 Corki.
Conclusion
Analyze the available data and learn how to beat Corki. To master Corki take also a look on Corki builds and runes for Corki. Good luck, Summoner!