How to counter Rek'Sai
Let us take a look on how to beat Rek'Sai. We will look at champions who are strong against Rek'Sai and who are weak against Rek'Sai. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Jungle champion who counters Rek'Sai. You can already win in champion select with the right counters for Rek'Sai. If you want to know how to counter Rek'Sai then you need to take into account the best picks vs Rek'Sai and the worst picks vs Rek'Sai. This Rek'Sai counter data is based on matches played by Pros on patch 14.12 and previous ones. It is important to see whether Pros counter picked Rek'Sai or whether they played as Rek'Sai.
Champions used by Pros to counter pick Rek'Sai
Looking at the matches from Pros (see table Pros counter picked Rek'Sai) we can see that they picked 
Lee Sin most often to counter pick Rek'Sai. The matchup Lee Sin vs Rek'Sai occured 69 times. Rek'Sai has 33 wins and 36 losses in this matchup. Learn how to counter pick Lee Sin to get another perspective on the data.
Lane outcome for Rek'Sai
When looking at Rek'Sai 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 Rek'Sai. The lane outcome of Rek'Sai consists of three outcomes: Win, Loss or Draw. Let's take a look of Rek'Sai 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 Rek'Sai.
Conclusion
Analyze the available data and learn how to beat Rek'Sai. To master Rek'Sai take also a look on Rek'Sai builds and runes for Rek'Sai. Good luck, Summoner!