1) All Chance (highest risk): These are games that are basically all left to chance and no skill is involved such as dice or roulette.
2) Mostly Chance but some Skill: These are games like poker or spades where there is some skill involved but you are at the mercy of the cards dealt.
3) Mostly Skill but some Chance: These are games or sports where it is mostly the player's skill that controls the outcome such as golf or bowling where each player plays against the game with the opponent facing the same game independently and under the same conditions. In these games you can't directly affect the opponent's performance. Could also be true for a video games where you play against the computer and not head to head against an opponent.
4) All Skill (lowest risk): These are games where you play head to head against an opponent and can affect the opponent's performance and you both are playing under the same conditions. These are games like chess and pool or sports like basketball.
This is just a theory on how risky betting is in some games or sports. This doesn't include betting on sports or horse racing where you are just an observer. What do you think?