Betting would be making a wager, as in saying if this team wins, you get this in a game of chance.
The video game is still a game of chance, but giving a reward is not a bet. MO defines gambling as:
(4) "Gambling", a person engages in "gambling" when he stakes or risks something of value upon the outcome of a contest of chance or a future contingent event not under his control or influence, upon an agreement or understanding that he will receive something of value in the event of a certain outcome. Gambling does not include bona fide business transactions valid under the law of contracts, including but not limited to contracts for the purchase or sale at a future date of securities or commodities, and agreements to compensate for loss caused by the happening of chance, including but not limited to contracts of indemnity or guaranty and life, health or accident insurance; nor does gambling include playing an amusement device that confers only an immediate right of replay not exchangeable for something of value. Gambling does not include any licensed activity, or persons participating in such games which are covered by sections 313.800 to 313.840; https://law.justia.com/codes/missouri/2011/titlexxxviii/chapter572/section572010
You are free to give a reward to the top player since you are not betting on an outcome, but giving a prize.
That applies to your state only however, if you have players from multiple states and countries you would have to comply with the laws in those areas too.