How to counter Zac
Let us take a look on how to beat Zac. We will look at champions who are strong against Zac and who are weak against Zac. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Jungle champion who counters Zac. You can already win in champion select with the right counters for Zac. If you want to know how to counter Zac then you need to take into account the best picks vs Zac and the worst picks vs Zac. This Zac counter data is based on matches played by Pros on patch 13.24 and previous ones. It is important to see whether Pros counter picked Zac or whether they played as Zac.
Champions used by Pros to counter pick Zac
Looking at the matches from Pros (see table Pros counter picked Zac) we can see that they picked 
Lee Sin most often to counter pick Zac. The matchup Lee Sin vs Zac occured 25 times. Zac has 15 wins and 10 losses in this matchup. Learn how to counter pick Lee Sin to get another perspective on the data.
Lane outcome for Zac
When looking at Zac 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 Zac. The lane outcome of Zac consists of three outcomes: Win, Loss or Draw. Let's take a look of Zac 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 Zac.
Conclusion
Analyze the available data and learn how to beat Zac. To master Zac take also a look on Zac builds and runes for Zac. Good luck, Summoner!