The games that really test the skills and strategy are sports betting because you have to understand some skills and strategies to play. And the second game I will say is poker game. In poker game you use your skills over others who are not good with it. Though there are some skills slot games and crash games also use some very little skills of the Play which the player must know when to hit the play button.
If you want to talk about completely skill in gambling, it should not be sports betting because it has a mixture of skill and luck although skills play a dominant role. What I consider entirely based on skill is games like poker because to win in poker, you must have the skill and not rely only on luck. You cannot make any serious progress in poker when you rely on luck.
I would go for poker as well. You need long years of experience and skills for you to be a professional and at the same time successful in this career. Whereas, in sportsbetting, it is true that you also need the luck factor in order for you to win the bet. In poker, you rely mostly in yourself.