The pronoun that takes the place of the noun 'game' is it.

Example: What time does the game start? It starts at two.

The correct form is "Who do you favor in the football game?". The pronoun "who" is functioning as the subject of the sentence.

The pronoun "who" is the subjective form.

The pronoun "whom" is an objective pronoun.

Using an object pronoun, the sentence would be:

'The game wardens noticed it.'

The word 'what' is used as an interrogative pronoun to ask a question and as relative pronoun to introduce a relative clause. Examples:

interrogative pronoun: What time does the game start?

relative pronoun: She didn't say what movie they saw.