One of the easiest is dice. You can easily manage the risks by lowing the win amount while learning it. Also, it's quite transparent that makes dice a good choice as an option to begin gamble with.
I agree with you on that, I think OP should ask what's the easiest online games rather than the safest because there is no safe in gambling, regardless of the amount we are gambling, we are risking money on that, and we could possibly lose more if we loss control along the way.
This is a good point because there is no guarantee as such that you would have higher chances of winning in one game compared to other . like you play slot games, roulette, poker etc no one can give guarantee of winning it. Easiest would be dice I guess or roulette also for that matter as you do not have to strain much of your brain to bets on this.
But still the brain will always be required to work harder even though you use a strategy about lowering the amount or odds, because when you don't use your brain even though you play it safe but in gambling there is always a risk of defeat. So, when you have a fixed brain in good condition in the sense of a good mindset, then you will also be able to control your emotions during betting.