Most colleagues mentioned blackjack, but I think that poker is more skill based than blackjack. The good thing is that, until you gain enough experience to feel confident, you can play free games.
Anyway, I don't think that there is a "least risky" game. There are games more based on skill than others, and games less addictive than others, but risk is inherent to gambling and odds will be (almost) always against you. So try to play for fun, not for money.
What may be considered a risk to one gambler, may be considered as money problems to others, because the game of gambling no matter the skill level or risk level, is dependent on how much the gambler is willing to lose before winning or throughout the course of their game.
I enjoy gambling online when I play easy games like 'spin d bottle', blackjack and roulette. As for skillset required, I think I just use my discretion to figure out the best ways to play and win and if I have enough cash won or in my wallet, I can just dance around the games and have fun while doing so until I get unlucky several times and have gotten to my limit for the day.