A game

Or A-game,

noun

Informal.

1. a person’s best effort, abilities, or achievement.

   The candidate brought her A game to the debate.

'A' game

noun

1. informal,  one's best possible performance, esp in the phrase to bring or take one's 'A' game

Word History and Origins

Origin of A game¹

In reference to a sports contest, with the letter A indicating peak performance