Other forms: crackerjacks
Someone who's a crackerjack is really good at what they do. A crackerjack of a magician will amaze you with every trick she performs.
You can use the word crackerjack for any excellent thing or person — you might say, for example, that your new car is a crackerjack or compliment your friend on her crackerjack of a performance at the school talent show. Crackerjack is an informal word that can be a noun or an adjective — and it's also the name of the caramel-covered popcorn and nuts you might buy at a baseball game.