How to counter Maokai
Let us take a look on how to beat Maokai. We will look at champions who are strong against Maokai and who are weak against Maokai. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Support champion who counters Maokai. You can already win in champion select with the right counters for Maokai. If you want to know how to counter Maokai then you need to take into account the best picks vs Maokai and the worst picks vs Maokai. This Maokai 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 Maokai or whether they played as Maokai.
Champions used by Pros to counter pick Maokai
Looking at the matches from Pros (see table Pros counter picked Maokai) we can see that they picked 
Poppy most often to counter pick Maokai. The matchup Poppy vs Maokai occured 38 times. Maokai has 20 wins and 18 losses in this matchup. Learn how to counter pick Poppy to get another perspective on the data.
Lane outcome for Maokai
When looking at Maokai 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 Maokai. The lane outcome of Maokai consists of three outcomes: Win, Loss or Draw. Let's take a look of Maokai 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 Maokai.
Conclusion
Analyze the available data and learn how to beat Maokai. To master Maokai take also a look on Maokai builds and runes for Maokai. Good luck, Summoner!