How to counter Zilean
Let us take a look on how to beat Zilean. We will look at champions who are strong against Zilean and who are weak against Zilean. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Zilean. You can already win in champion select with the right counters for Zilean. If you want to know how to counter Zilean then you need to take into account the best picks vs Zilean and the worst picks vs Zilean. This Zilean counter data is based on matches played by Pros on patch 15.11 and previous ones. It is important to see whether Pros counter picked Zilean or whether they played as Zilean.
Champions used by Pros to counter pick Zilean
Looking at the matches from Pros (see table Pros counter picked Zilean) we can see that they picked 
Sylas most often to counter pick Zilean. The matchup Sylas vs Zilean occured 4 times. Zilean has 3 wins and 1 losses in this matchup. Learn how to counter pick Sylas to get another perspective on the data.
Lane outcome for Zilean
When looking at Zilean 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 Zilean. The lane outcome of Zilean consists of three outcomes: Win, Loss or Draw. Let's take a look of Zilean 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 Zilean.
Conclusion
Analyze the available data and learn how to beat Zilean. To master Zilean take also a look on Zilean builds and runes for Zilean. Good luck, Summoner!