How to counter Illaoi
Let us take a look on how to beat Illaoi. We will look at champions who are strong against Illaoi and who are weak against Illaoi. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Top champion who counters Illaoi. You can already win in champion select with the right counters for Illaoi. If you want to know how to counter Illaoi then you need to take into account the best picks vs Illaoi and the worst picks vs Illaoi. This Illaoi 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 Illaoi or whether they played as Illaoi.
Champions used by Pros to counter pick Illaoi
Looking at the matches from Pros (see table Pros counter picked Illaoi) we can see that they picked 
Jax most often to counter pick Illaoi. The matchup Jax vs Illaoi occured 6 times. Illaoi has 4 wins and 2 losses in this matchup. Learn how to counter pick Jax to get another perspective on the data.
Lane outcome for Illaoi
When looking at Illaoi 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 Illaoi. The lane outcome of Illaoi consists of three outcomes: Win, Loss or Draw. Let's take a look of Illaoi 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 Illaoi.
Conclusion
Analyze the available data and learn how to beat Illaoi. To master Illaoi take also a look on Illaoi builds and runes for Illaoi. Good luck, Summoner!