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.
According to the current result of the poll, Dice and Roulette are the safest games for newbies but I don't agree with the majority of the voters. Roulette has the 47% of probability of hitting odd side of numbers and the rest will go to the house edge. Sports betting is not based on the pure luck and house edge is lower than other games/bets. So, the easiest and skill required gambling should be sports betting, IMHO.