This depends on whether you mean easiest to get a win or easiest to win over time.
For example, it’s easy to play roulette or other table games at a casino site, bet on red or black, and you have just under a 50/50 chance of winning on each spin of the wheel. The law of averages states that it won’t be long before you have a win. But you’ll likely lose over time because every online casino is clever at making that happen.
The easiest to win over time is by playing tournaments or other forms of gaming requiring skill, simply because they require skill, and if you have it, you will have an advantage over other players.
The easiest to win if playing casino games is probably 3 card poker or video poker, but even then, the odds are still against you.