For me it is card games, I am very familiar with card games especially with poker.
Familiarity with a game and chance of winning the game is not always same if the odds are stacked against you. Gambling is not something like playing tennis where you can practice your game and hone it. Gambling is a game of luck/chance and so even in Poker which is an EV+ game can be very devastating for a winner due to that luck factor. None of the games in gambling is completely off luck, it would not be a gambling game then.
But it all depends on every gambler since the criteria for choosing the safest online gambling is what game you are comfortable with and familiar with.
You will be comfortable to bet on a game you are familiar with. But this does not mean you will win everytime or that it is a safe game to play. Reward comes with the risk. If you dont take the risk you dont get a profit.