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

Example Sentences

In my head, I thought it was a game.

From Los Angeles Times

The use of Vesia in such a lopsided game spoke to how little Roberts wanted to use any of his other relievers in a game of this magnitude, but the fiery left-hander looked like a rubber band that had been stretched out too many times.

From Los Angeles Times

They face the Cleveland Browns at Tottenham on Sunday, a game which features two of the NFL's top-10 players, external - Vikings receiver Justin Jefferson and Browns defensive end Myles Garrett.

From BBC

Going into week five, the Jets are one of three teams yet to win a game.

From BBC

We just loved the idea that this face represented a fresh start for someone — a guy who had all the talent, who spent his entire life and gave everything he had to a game, but his face was the thing preventing him from getting back on that field.

From Los Angeles Times