Wordorigins.org

Crackerjack meaning excellent or superb is an Americanism dating to 1888. Crack-a-jack is a variant. The origin of the term is obscure. From New York Times of 23 April 1888:

The Canadian owner, Mr. Forbes, has Rowland, Brait, and several others in McDonald’s stable, and Charles Miller has four, two of them being 2-year-olds which he expects will become “Cracker Jacks.”

The brand name for the caramel popcorn product comes from the slang usage. The name was trademarked in 1896.

(Sources: Oxford English Dictionary, 2nd Edition; ADS-L; ProQuest Historical Newspapers The New York Times)